Fix a typo in getprop. (#562)

diff --git a/mobly/controllers/android_device_lib/adb.py b/mobly/controllers/android_device_lib/adb.py
index 74d4432..9551c68 100644
--- a/mobly/controllers/android_device_lib/adb.py
+++ b/mobly/controllers/android_device_lib/adb.py
@@ -306,7 +306,7 @@
             doesn't exist.
         """
         return self.shell(
-            ['getprop', 'prop_name'],
+            ['getprop', prop_name],
             timeout=DEFAULT_GETPROP_TIMEOUT_SEC).decode('utf-8').strip()
 
     def has_shell_command(self, command):
diff --git a/tests/mobly/controllers/android_device_lib/adb_test.py b/tests/mobly/controllers/android_device_lib/adb_test.py
index 6f4e276..ea9dd16 100755
--- a/tests/mobly/controllers/android_device_lib/adb_test.py
+++ b/tests/mobly/controllers/android_device_lib/adb_test.py
@@ -174,8 +174,8 @@
         self._mock_execute_and_process_stdout_process(mock_popen)
         mock_handler = mock.MagicMock()
         mock_popen.return_value.communicate = mock.Mock(
-            return_value=(unexpected_stdout, MOCK_DEFAULT_STDERR.encode(
-                'utf-8')))
+            return_value=(unexpected_stdout,
+                          MOCK_DEFAULT_STDERR.encode('utf-8')))
 
         err = adb.AdbProxy()._execute_and_process_stdout(
             ['fake_cmd'], shell=False, handler=mock_handler)
@@ -448,6 +448,11 @@
         with mock.patch.object(adb.AdbProxy, '_exec_cmd') as mock_exec_cmd:
             mock_exec_cmd.return_value = b'blah'
             self.assertEqual(adb.AdbProxy().getprop('haha'), 'blah')
+            mock_exec_cmd.assert_called_once_with(
+                ['adb', 'shell', 'getprop', 'haha'],
+                shell=False,
+                stderr=None,
+                timeout=5)
 
     def test_forward(self):
         with mock.patch.object(adb.AdbProxy, '_exec_cmd') as mock_exec_cmd: