SystemUI: Show bluetooth battery level

* Somewhat inspired by change committed by Gavin Ni <gisngy@gmail.com>
  back in cm-13.0 days (see commit 88e7a6c7). Since then completely
  rewritten using BluetoothDevice API introduced in Oreo, with new
  drawables meant to be used with 0-9 battery level range.

Co-authored-by: Luca Stefani <luca.stefani.ge1@gmail.com>
Change-Id: I34344907711b7b9fc4c1d15d4516bd53bf30ebb2