Don't initialize Firebase for tests

[bherbst]

One somewhat common source of test flakiness that I have observed is Firebase failing to initialize in our unit test, usually with a message like this:

java.util.concurrent.RejectedExecutionException: Task java.util.concurrent.FutureTask@3a918bde[Not completed, task = java.util.concurrent.Executors$RunnableAdapter@2975e4ef[Wrapped task = com.google.android.gms.internal.crash.zzf@46f1dde8]] rejected from java.util.concurrent.ThreadPoolExecutor@a5db1dd[Terminated, pool size = 0, active threads = 0, queued tasks = 0, completed tasks = 1]

Really, we shouldn't be initializing Firebase and using it in our tests anyway.

The vast majority of this dependency comes from our TbaLogger, which logs everything to Firebase. My proposal is that we switch to Timber and use a different Tree for tests that doesn't report to Firebase.