SystemUI: improve battery saver tile

- Fix the tile not showing, the javadoc states that it will return 0 if
  profiles are not supported, but it was actually returning -1.

- Now disables itself when the device is plugged in to reflect to the user
  that they cannot toggle it on right now.

- Don't wait for power manager to update its internal state before
  updating the visual tile state.

Change-Id: I8ab6adb2bfa728a242368294c1f066d468a86f09
Signed-off-by: Roman Birg <roman@cyngn.com>