Saver: PowerManager call to set low power mode.

- Add an explicit power manager call to set the low power mode state,
  instead of trying manage everything around a single setting.
- When low-power mode is triggered by falling below the configured
  threshold, it does not update the setting.
- The "is-enabled" api returns setting || below configured trigger.
- Move the snooze management into the new api call.
- Callers (sysui + settings) updated to use the api instead of the
  setting.
- Handles the case where the level does an unpowered leap out of the
  low battery level. (Possible if powered in-between while the device
  is off)

Bug:17460535
Change-Id: Ic030504c9cad9868a7137abbe837b170da37852b