[pm] revert lock changes to settings write

This reverts ag/25983279, which introduced a behavior change in the
locking of writeSettings. As a result of that CL, the disk writes were
not guarded by "synchronized (mLock)" which may cause issues if the
settings are being modified at the same time as the writes.

BUG: 345278249
Test: presubmit
Change-Id: I3c855c986bb7bc9b7ab220e416305d562c621a5b