Fix charging indication.

This commit fixes a number of issues in the new battery drawing code:
- Use the BatteryTracker instances consistently
- Don't indicate charging (by displaying the bolt or the circle
  animation) if the device is connected to a charger, but not actually
  charging, e.g. because of the charger not providing a sufficient
  amount of current.
- Fix some style issues (variable naming)

Change-Id: I957c8c525310170a3d4480047fbf6e92533c0296