Support volume and looping when playing alarm ringtones

Pre-M, playback is done via MediaPlayer. The READ_EXTERNAL_STORAGE
permission is granted at install time on pre-M platforms, so
playing back custom ringtones never fails.

M+, playback is done via Ringtone. No special permissions are
required to play custom ringtones.

Bug: 20273223
Change-Id: I4313e49c4555e820afb4ed0361f81c0dab90134e