[Cashtab] Automatically update android app version from package dot json

Summary: We want to keep the android app in sync with the webapp and the extension. Pull the version number from package.json for release builds.

Test Plan: ./build-android-release.sh, version matches

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18950