Setup Firebase Functions for Token Issuer

This CL enables the app to request debugging access tokens from the
remote web service using Firebase Functions. Only signed-in users can
request tokens.

Bug: 173732645
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: I162f95ac2a74b01995f92e63a11247a27217dddc
8 files changed