Play the low-battery sound only at certain discharge events.

That is, only play it when the device is running on battery
and the remaining charge level descends into a new
low-battery regime:

* when the device hits the first threshold, for example 15%
  (the dialog is first shown at this time)
* when the device hits the second threshold, e.g. 4% (the
  dialog will be redisplayed if necessary)
* when the device has been charging but AC power is removed,
  returning the device to a low-battery regime (the dialog
  will be redisplayed if necessary)

The sound will no longer be replayed as the battery level
descends within these regimes (say, 4% --> 3%).

Bug: 4981280
Change-Id: I049d60f39ae556241a23f8664e61be3d70d937e5