GCM: Wake with exact guarantees for heartbeat

This changes how the alarms for heartbeat pings are scheduled. Instead
of a repeating, inexact alarm that may be delayed (at least up to 15
minutes), either an exact alarm (below Android 4.4) or an interval is used
which ensures that the alarm fires between half the configured interval
and the configured interval. This interval allows the OS to optimize
alarms a bit. For Android 6.0 further adjustments are probably
necessary.