Experimental: in-app updates

[cooltey]

https://developer.android.com/guide/playcore/in-app-updates/kotlin-java

Follow the top-voted answer steps to test the APK locally: https://stackoverflow.com/questions/56087064/how-can-i-test-in-app-updates-in-android

Demo: https://youtu.be/E8SkgfyQT9g

[dbrant]

Very nice! Some initial comments:

• Obviously this won't do anything in builds released to other stores, e.g. Samsung, Huawei, etc. But we should make sure in those cases it doesn't have any unusual side effects.

• In the case of F-Droid, bundling a Google library will not be acceptable. Ideally this should be split off into a class in our extras folder, similar to how we handle Firebase Messaging.

[cooltey]

Close for now but keep the branch for future reference.