[Cashtab] Add a script to build a signed release

Summary: Add a script with README instructions for building a signed prod release

Test Plan: ./build-android-release.sh ... though it only works if you also follow the readme instructions and create the key and the properties file

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18931