Fix NPE in AlarmManager.cancel(...)

If AlarmManager.cancel is called before any component has had a chance
to register an alarm, sWrappers will not have been set, as it is only
initialized in setImpl.

When cancelling an alarm and sWrappers does not exist, log a warning and
return early.

BUG: 27972657
Change-Id: Ied10719ba1d154f97eecfc32f429fd9ce39bf747