Reduce observers in SystemUI: BatteryBarController

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 instances.

On my Nexus 10 it reduced the observer count from 15 to 3

Change-Id: Ibdd6fca75e5f0245b2897c9d5aebb5a0961cc351
Signed-off-by: Dirk Rettschlag <dirk.rettschlag@gmail.com>