[Cashtab] Improved android app scripts

Summary:
Maintain separate "flavors" (android term) for dev and prod builds. This allows us to build dev apks to test diffs. We cannot build an app with the same ID as prod without making a new prod build that overwrites the existing prod app. So this is useful for testing.

Now that we have more scripts that do more things, we organize them in scripts/android

Test Plan: Run the scripts. I did this locally to make dev and prod builds that work and have distinct app ids.

Reviewers: #bitcoin_abc, johnkuney

Reviewed By: #bitcoin_abc, johnkuney

Differential Revision: https://reviews.bitcoinabc.org/D19207