Overhaul BatteryController and related code cleanup (1 of 2)

Summary:
Clean up statusbar battery text label implementation. Also, start
unwinding battery controls in ui controllers, as it is not needed.

Depends on D438

Change-Id: I5f6dfcb2b191a4cb1cb2e8e04e38db9c99be6f53

Test Plan: test and merge

Reviewers: synergy

Reviewed By: synergy

Differential Revision: http://review.codefirex.com/D442