Persist snoozed notifications after restart

The state of snoozed notifications is lost after restart. To prevent
snoozed notifications from sending notifications, we intercept posted
notifications and re-snooze them if they were supposed to be snoozed
before restart/turning off. We store that state in the
notifications_polixy.xml file. (usually in /data/system)

Bug: 120617189
Test: atest SnoozeHelperTest NotificationManagerServiceTest
Change-Id: I3a19ad2f0ba98ab385a0ff12a5c7affba6e4f1e5