Remove dependency on Firebase Messaging

[marcprux]

Currently, we depend on Firebase Messaging for handling push notifications: https://github.com/skiptools/skip-notify/blob/3ad44e165defe435cf6d618d9fe985d76ea40e02/Sources/SkipNotify/Skip/skip.yml#L24C28-L24C66

        - 'implementation("com.google.firebase:firebase-messaging")'

But this dependency could be removed if we instead interface directly with the "com.google.android.gms" service on the device (if present), like was done at christianrowlands/ninja-chat@1abb22b

This opens the possibility of supporting devices that do not have Google Mobile Services available (AOSP builds, Chinese ROMs and other non-Android Certified devices, etc.), as well as enabling flexible support for multiple notification services (as discussed in #2).