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.

Change-Id: I611ec77174ab45315ccbee2952bdbc2c9a9cd954
Fix: 37227609
Test: make RunSettingsRoboTests
5 files changed