Synchronize access to notif settings

And fix an instance where we were calling into PM with a
lock held.

Flag: EXEMPT bugfix
Bug: 343447769
Test: PreferenceHelperTest
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:490c3ee65e26f0a3db5819510eef2355a149dc4b)
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:83e6ba7dc06fbd852da588082b33246fa4b70b64)
Merged-In: I1fec0503bf347f176305744b77ad27d78fe8161c
Change-Id: I1fec0503bf347f176305744b77ad27d78fe8161c