Initialize ContentObserver in startObserving().

This removes a race condition where ContentObserver is initialized
while mHandler is null which causes the ContentObserver#onChange() callback to run on a different thread and can cause an NPE.

Bug: 246843362
Test: manual
Test: atest BrightnessSynchronizerTest
Change-Id: Ib8e192c3961390ece721fa473d5c09323c30bafe