Improve shuffle and skip track

Improve operation of skip back a track and keep shuffle order between app runs.

Also removed Proguard from release build to possibly help in some installations.

Bug fix

Fix crash in edge case where Android’s media manager has a genre with no songs/tracks.

Bug fix

Fix crash prev or next pressed when denied permission to access song storage.

Bug Fix

• Added privacy policy.
• Update Gradle version.
• Revise logic on previous track button: Previous track button now uses the shuffle list rather than tracking play history. This allows the previous button to be operational even if there was no previous track played (initial startup) and eliminates need for special handling to avoid putting previous track into play history.
• Fix crash when starting notification on newer versions of Android (tested on 8.1)

Development environment update

• Update build environment.