Remove Google software from our build

(At least) firebase-iid and firebase-messaging are defined in reflex-platform/android/build.gradle.nix and reflex-platform/android/defaults/deps.nix

Since we want to publish on F-Droid (#90), we should removed that (and all other non-open Google stuff):

https://f-droid.org/en/docs/Inclusion_Policy/

Notably:

• We cannot build apps using Google’s proprietary “Play Services”. Please talk to upstream about an untainted build flavor (either using microg or removing Non-Free dependencies completely).
• We cannot build apps using proprietary tracking/analytic dependencies like Crashlytics and Firebase. Please talk to upstream about an untainted build flavor (either using a FLOSS analytics software like ACRA or by removing Non-Free dependencies completely).
• We cannot build apps using proprietary ad libraries. We have nothing against advertisments (see Anti-Features), but they must be provided by a FLOSS-compatible way.
• We cannot build apps requiring Non-Free build tools, including Oracle’s JDK or some pre-release toolchains.