Reduce observers in SystemUI: KeyButtonView

This is based on the work by Jens Doll for the StatusBarIconView

The ContentObserver class will be a singleton, so there is only
one observer for all buttons.

On my Nexus 10 it reduced the observer count from 48 to 4.

This version fixes the funky glow effect.

Change-Id: Ie242e3771c41f6010ac12246372107a2a3e165cd
Signed-off-by: Dirk Rettschlag <dirk.rettschlag@gmail.com>