Add get_service_alias_by_class in Android service manager.

This is useful for looking up how many instances of a service has
been registered, and find out their aliases.

Util functions may depend on services like `uiautomator`. Users tend
to follow the anti-pattern of attempting to register the service
again under the name the util expects. This causes duplication.
This API can be used to clean things up.

mobly/controllers/android_device_lib/service_manager.py

@@ -122,6 +122,24 @@ def for_each(self, func):
       ):
         func(self._service_objects[alias])
 
+  def get_service_alias_by_class(self, service_class):
+    """Gets the aslias name of a registered service.
+
+    Note the same class can be registered multiple times with different alias
+    names. So this returnes a list.
+
+    Args:
+      service_class: class, the class of a service type.
+
+    Returns:
+      list of strings, the aliases the service is registered with.
+    """
+    aliases = []
+    for alias, service_object in self._service_objects.items():
+      if isinstance(service_object, service_class):
+        aliases.append(alias)
+    return aliases
+
   def list_live_services(self):
     """Lists the aliases of all the services that are alive.
 

tests/mobly/controllers/android_device_lib/service_manager_test.py

@@ -469,6 +469,14 @@ def test_resume_services_non_existent(self):
     with self.assertRaisesRegex(service_manager.Error, msg):
       manager.resume_services(['mock_service'])
 
+  def test_get_alias_by_class(self):
+    manager = service_manager.ServiceManager(mock.MagicMock())
+    manager.register('mock_service1', MockService, start_service=False)
+    manager.register('mock_service2', MockService, start_service=False)
+    manager.start_services(['mock_service2'])
+    aliases = manager.get_service_alias_by_class(MockService)
+    self.assertEqual(aliases, ['mock_service1', 'mock_service2'])
+
 
 if __name__ == '__main__':
   unittest.main()