Setup Firebase Authentication

This CL enables email/password based authentication.
Instrumentation tests cover 96% of code in ui/login.

Bug: 173732020
Test: Place google-services.json in app/
Test: Get credentials from valentine/#/show/1611079519238994
Test: Set envvars DRC_TEST_EMAIL and DRC_TEST_PASSWORD
Test: ./gradlew connectedAndroidTest
Test: ./gradlew createDebugCoverageReport

Change-Id: I30d0246bd8640cc75c03dd3948a4329057e343f7
8 files changed