TESTING-Make APK (initially) available for testing and feedback

[chrisdebian]

In order to encourage user testing and feedback (CI/CD), the application should be made available.

This will probably be as an APK, initially, and downloadable from Git. I suspect this will always be available from Git, but a separate issue will cover releases from app stores.

Set an informal target date for an initial APK, in the spirit of 'Release early, release often'.

@ Nick, this isn't aimed at you; this will be for when we get more contributors onboard.

Chris