Include fastboot devices when validating controller creation. (#791)

This allows running Mobly tests against devices that are in fastboot mode 
without manually rebooting them first.
diff --git a/mobly/controllers/android_device.py b/mobly/controllers/android_device.py
index d786738..843d8eb 100644
--- a/mobly/controllers/android_device.py
+++ b/mobly/controllers/android_device.py
@@ -149,7 +149,8 @@
     serials: list of strings, the serials of all the devices that are expected
       to exist.
   """
-  valid_ad_identifiers = list_adb_devices() + list_adb_devices_by_usb_id()
+  valid_ad_identifiers = (list_adb_devices() + list_adb_devices_by_usb_id() +
+                          list_fastboot_devices())
   for serial in serials:
     if serial not in valid_ad_identifiers:
       raise Error(f'Android device serial "{serial}" is specified in '