DeskClock: Add feature to play random song as an alarm.

Change-Id: I38c346beb0746fcebd224e9641e2d397183ba9a2

(cherry picked from commit f8d1754d3cc68cc5abc6cf2d4b79a1dfa374c104)
DeskClock: Fix NullPointerException in AlarmClockFragment

RingtoneManager.getRingtone could return null so add a check for that

Change-Id: Ic3ec042e7c3fe02eee6388a469f19624d90fdddd

(cherry picked from commit 5081fd98bfc35a511fe4eb3478e75cfc1e7b6347)