SystemUI: Show bluetooth battery level when available

* Somewhat inspired by change committed by Gavin Ni <gisngy@gmail.com>
  back in cm-13.0 days (see commit 88e7a6c). Since then completely
  rewritten using BluetoothDevice API introduced in Oreo, with new
  drawables meant to be used with 0-9 battery level range.

Change-Id: I6179bfd41e033591534e8cf3c6adc98ce715a13d
11 files changed