Fix some more snoozing bugs

- Make all time based snoozing use the same time frame
(current millis, not elapsed walltime)
- Add synchronization to snoozing
- Update when entries are and are not removed from data structures
to avoid NPEs

Test: atest, and manual, repeated snooze and unsnooze
Fixes: 150377159
Change-Id: Ic91ff7ac0bd1b3635b35267bdbc64d3ee614c8ab