Android kokoro testing should be added.

Looking at #926, the [README](../blob/master/README.md) for the repo, and [.kokoro](../tree/master/.kokoro): it appears that testing using the [Android Emulator](https://medium.com/@AndreSand/android-emulator-on-docker-container-f20c49b129ef) [official](https://developer.android.com/studio/run/emulator) and [firebase test lab](https://firebase.google.com/docs/test-lab/android/command-line) has been left out, it's probably a good idea to run these on presubmit and nightly.