Trim strings added to persistent snoozed notification storage.

This is a backport of ag/20581190 and includes the fix in ag/20778075.
Note that on this branch, clearData doesn't seem to actually clear persistent storage.

Fix: 258422365
Test: atest NotificationManagerServiceTest SnoozeHelperTest
Change-Id: If7c7db6694330ffbac551d044efadb26219fe17f
Merged-In: I5a2823f10053ea8c83c612a567d6d4f1b6af23e7
Merged-In: Ie809cb4d648a40622618e0fb374f36b6d8dc972a
(cherry picked from commit on googleplex-android-review.googlesource.com host: 2a01a489)
Merged-In: If7c7db6694330ffbac551d044efadb26219fe17f