Fix crashloop in CarStatusBar.

System ui seems to be undergoing a refactor to bring in
dependency injection, in the process, mBatteryController
in CarStatusBar was not being set, causing a NPE.

Since it looks like the work is still ongoing, this is
just a spot fix for the crash, once the dependency injection
migration is complete, we can reexamine the CarStatusBar
implementation.

Bug: 34633087
Test: Verified that system booted correctly.
Change-Id: Idd3e0286fd87196ae572349904d5bef744d54353