Add keeps for strict resource shrinking. (#3663)

Fixes: b/221423396

Author: vkryachko

firebase-common/src/main/res/raw/keep.xml

@@ -0,0 +1,4 @@
+<?xml version="1.0" encoding="utf-8"?>
+<resources xmlns:tools="http://schemas.android.com/tools"
+    tools:keep="@string/google_app_id,@string/gcm_defaultSenderId,@string/google_api_key,@string/firebase_database_url,@string/ga_trackingId,@string/google_storage_bucket,@string/project_id"
+    tools:shrinkMode="strict" />