Fix flake8 warning (#741) (#745)

diff --git a/mobly/controllers/android_device.py b/mobly/controllers/android_device.py
index dc16180..2c2375c 100644
--- a/mobly/controllers/android_device.py
+++ b/mobly/controllers/android_device.py
@@ -123,7 +123,7 @@
   for ad in ads:
     try:
       ad.services.stop_all()
-    except:
+    except Exception:
       ad.log.exception('Failed to clean up properly.')
 
 
diff --git a/mobly/controllers/android_device_lib/event_dispatcher.py b/mobly/controllers/android_device_lib/event_dispatcher.py
index ee28000..26db911 100644
--- a/mobly/controllers/android_device_lib/event_dispatcher.py
+++ b/mobly/controllers/android_device_lib/event_dispatcher.py
@@ -62,7 +62,7 @@
       event_name = None
       try:
         event_obj = self._sl4a.eventWait(50000)
-      except:
+      except Exception:
         if self.started:
           print("Exception happened during polling.")
           print(traceback.format_exc())
@@ -269,7 +269,7 @@
       raise IllegalStateError("Dispatcher needs to be started before popping.")
     deadline = time.perf_counter() + timeout
     while True:
-      #TODO: fix the sleep loop
+      # TODO: fix the sleep loop
       results = self._match_and_pop(regex_pattern)
       if len(results) != 0 or time.perf_counter() > deadline:
         break
@@ -292,7 +292,7 @@
         if q:
           try:
             results.append(q.get(False))
-          except:
+          except Exception:
             pass
     self.lock.release()
     return results
diff --git a/mobly/controllers/android_device_lib/jsonrpc_client_base.py b/mobly/controllers/android_device_lib/jsonrpc_client_base.py
index e28f5a1..39266dc 100644
--- a/mobly/controllers/android_device_lib/jsonrpc_client_base.py
+++ b/mobly/controllers/android_device_lib/jsonrpc_client_base.py
@@ -54,7 +54,6 @@
 import abc
 import json
 import socket
-import sys
 import threading
 
 from mobly.controllers.android_device_lib import callback_handler
diff --git a/mobly/controllers/android_device_lib/jsonrpc_shell_base.py b/mobly/controllers/android_device_lib/jsonrpc_shell_base.py
index 24488a2..3129f59 100755
--- a/mobly/controllers/android_device_lib/jsonrpc_shell_base.py
+++ b/mobly/controllers/android_device_lib/jsonrpc_shell_base.py
@@ -32,14 +32,14 @@
 
     Must be implemented by subclasses.
     """
-    raise NotImplemented()
+    raise NotImplementedError()
 
   def _get_banner(self, serial):
     """Returns the user-friendly banner message to print before the console.
 
     Must be implemented by subclasses.
     """
-    raise NotImplemented()
+    raise NotImplementedError()
 
   def load_device(self, serial=None):
     """Creates an AndroidDevice for the given serial number.
diff --git a/mobly/controllers/android_device_lib/service_manager.py b/mobly/controllers/android_device_lib/service_manager.py
index 1885ac8..983cff7 100644
--- a/mobly/controllers/android_device_lib/service_manager.py
+++ b/mobly/controllers/android_device_lib/service_manager.py
@@ -15,7 +15,6 @@
 # TODO(xpconanfan: move the device errors to a more generic location so
 # other device controllers like iOS can share it.
 import collections
-import contextlib
 import inspect
 
 from mobly import expects
diff --git a/mobly/controllers/android_device_lib/services/logcat.py b/mobly/controllers/android_device_lib/services/logcat.py
index 112b3e8..cbd8e95 100644
--- a/mobly/controllers/android_device_lib/services/logcat.py
+++ b/mobly/controllers/android_device_lib/services/logcat.py
@@ -239,7 +239,7 @@
       return
     try:
       utils.stop_standing_subprocess(self._adb_logcat_process)
-    except:
+    except Exception:
       self._ad.log.exception('Failed to stop adb logcat.')
     self._adb_logcat_process = None
 
diff --git a/mobly/controllers/android_device_lib/sl4a_client.py b/mobly/controllers/android_device_lib/sl4a_client.py
index cfbc56b..725a61c 100644
--- a/mobly/controllers/android_device_lib/sl4a_client.py
+++ b/mobly/controllers/android_device_lib/sl4a_client.py
@@ -107,7 +107,7 @@
         # Be polite; let the dest know we're shutting down.
         try:
           self.closeSl4aSession()
-        except:
+        except Exception:
           self.log.exception('Failed to gracefully shut down %s.',
                              self.app_name)
 
@@ -126,7 +126,7 @@
     if self.ed:
       try:
         self.ed.clean_up()
-      except:
+      except Exception:
         self.log.exception('Failed to shutdown sl4a event dispatcher.')
       self.ed = None
 
@@ -140,7 +140,7 @@
         self.connect()
         started = True
         break
-      except:
+      except Exception:
         self.log.debug('%s is not yet running, retrying',
                        self.app_name,
                        exc_info=True)
diff --git a/mobly/controllers/android_device_lib/snippet_client.py b/mobly/controllers/android_device_lib/snippet_client.py
index 25adb85..bc831ec 100644
--- a/mobly/controllers/android_device_lib/snippet_client.py
+++ b/mobly/controllers/android_device_lib/snippet_client.py
@@ -129,7 +129,7 @@
       # If errors happen, make sure we clean up before raising.
       try:
         self.stop_app()
-      except:
+      except Exception:
         self._ad.log.exception(
             'Failed to stop app after failure to start and connect.')
       # Explicitly raise the original error from starting app.
@@ -204,7 +204,7 @@
     self._adb.forward(['tcp:%d' % self.host_port, 'tcp:%d' % self.device_port])
     try:
       self.connect()
-    except:
+    except Exception:
       # Log the original error and raise AppRestoreConnectionError.
       self.log.exception('Failed to re-connect to app.')
       raise jsonrpc_client_base.AppRestoreConnectionError(
@@ -313,7 +313,7 @@
       # considering any blank output line to be EOF.
       line = line.strip()
       if (line.startswith('INSTRUMENTATION_RESULT:') or
-          line.startswith('SNIPPET ')):
+         line.startswith('SNIPPET ')):
         self.log.debug('Accepted line from instrumentation output: "%s"', line)
         return line
       self.log.debug('Discarded line from instrumentation output: "%s"', line)