Introduce new lock to avoid lock-ordering issues.

Because of the way DMD was designed, technically the class lock is below
the syncroot in the lock ordering, meaning we can't call into any
general display classes while holding the class lock without creating
the possibility of deadlock. There's no real reason to use the class
lock in this situation though, so just introduce a local lock to
SensorObserver that can be above the syncroot and thus call into the
rest of the display stack at will.

Bug: 195713475
Test: atest DisplayModeDirectorTest
Change-Id: Ie32660ae3a3935f0fd4001ca73fecba1e4bbba7d