Enable FCM in the Expo client
Enable FCM in the Expo client

We are moving to FCM for Android notifications because Google has deprecated GCM and we have deliverability issues with GCM/SNS. This commit adds the FCM services config to AndroidManifest.xml for all apps and then sets the `FCM_ENABLED` to true by default. For standalone apps that don't have a `google-services.json` file, we set `FCM_ENABLED = false`.

Tested by building the Expo client locally and running the push notification test experience. Verified in the Postgres DB's `push_devices` table that the client registered using FCM, and sent a test notification with the notification dashboard to test notifications end-to-end.

Detached a project and verified that `FCM_ENABLED = false` was in the project (which didn't have FCM set up for standalone).

fbshipit-source-id: d2b787e

exponentjs/xdl,exponentjs/xdl,exponentjs/xdl