Android: let fullscreen use entire screen

API level 19 (Android 4.4) introduces "immersive" mode which lets
the app use the entire screen.

Change-Id: I12f6aebaf1303cdc5b6bfb51944e895351fa2406
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>