Add integration test checking functionality of Push

[jraska]

Motivation

• End to end Testing push integration with Firebase Cloud messaging is difficult as it requires interaction with Firebase. servers. and waiting for the push to appear on client.
• Android device which runs test can serve both as push message producer and. consumer. The process can be automated like this.

The test scenario

1. The device uses FCM API to send a push message to itself.
2. The device waits until it receives push with deep link. The deep link push implementation triggers certain screen change.
3. Test verifies the certain UI change did happen on the screen.

Implementation details

• The main class of interest is PushIntegrationTest. It implements the test scenario.
• We pass instrumentation runner argument FCM_API_KEY and use FCM HTTP API to trigger the push to itself using PushServerClient
• PushHandleModel became interface to be able to hook into push in tests.
• To idle Espresso properly, we hook into ServiceModel.Factory and we provide decorated version of PushHandleModel - TestPushHandleModel - this one makes the push idling resource Idle to allow Espresso screen verification.
• PushAwaitRule holds the logic for idling resource waiting for push.
• Tests are ignored if the FCM_API_KEY argument isn't provided. For example for local runs when we don't want to do this always.

Resolves #294

[jraska]

This PR was released with 0.22.1