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.

Change-Id: I3223a18fcb5d2c9c2e7de655c514ff751dcd345d
Signed-off-by: Dirk Rettschlag <dirk.rettschlag@gmail.com>