fixup! SystemUI: Show bluetooth battery level when available

The current update method is triggered in 2 cases:
  the SystemUI bluetooth controller changes device state
  the battery level changed intent is sent

Sadly the current SysUI controller doesn't handle battery level
changes, and is instead triggered in a few other cases ( ex audio
state changed ).
And since we use the data from SysUI controller for battery level
from changes in the global bt controller we may have 2 different
set of devices to work on. This could ( and does ) trigger
race conditions.

Instead add support battery level state changes within the SystemUI
controller to work on the same data set.

Change-Id: Ib57096fc2ebe151ac3860277a4db2319f230b514