Build guide for APK

Hi,

Since Trezor does not provide an APK for android users outside of Google Play, it would help to provide a guide for users to build their own APK.

The build docs in github give some hints, but there is no suitable guide for non-devs to build their own APK. Since non-dev users need to build the APK, Trezor should provide a guide for those users to build it themselves.