Listen to brigtness updates to update brightness summary.

- add content observer for brightness relates changes, and update the
brightness preference summary when changes occur.
- also check for VR mode and auto brightness mode when reading the
current brightness value.

Merged-In: I895d251e1dc12cb114ef00328eb8aa0075d77ab1
Change-Id: Ice549ffd708c2dad311a87b6a92bf1c8db426456
Fix: 37227609
Test: make RunSettingsRoboTests