Eliminate next-alarm-clock broadcast flapping

When multiple alarm clocks are scheduled at the same time, we would
flap among the alternatives for considering them the 'next upcoming
alarm clock', which in turn would generate [many] spurious broadcasts
about changes to the upcoming alarm situation.  This is now fixed;
once we have found the soonest upcoming alarm clock, we stick with
that one until it becomes unavailable, eliminating the spurious
broadcast traffic.

Bug 29501073

Change-Id: Ice1892490bb339e05fa8bd9d324fa1c6718b4942
(cherry picked from commit 76389c00)