Update Android java code (#7820)

Update Android java code (#7820)

Targets SDK 26 as required by the playstore.
Fixes screen auto-rotation closing game.
Hides on-screen navigation bar if present.

Update gradlew.
Fix display aspect on 18+/:9 displays (like a Samsung Galaxy S9).
Remove small app icons, not required.
Fix xml in unpacking activity.
Support Android permission: On Android 6.0+ you need to manually give write
permission (as required by google).
Background during unpacking (just a demo for now).
Material Design: no more Android 2 interface.
Immersive mode (Android 4.4+ - hide NavBar for fullscreen mode).