Framework: Volume panel cleanup (1/2)

- Hide notifications volume slider when linked with ringtone volume.
- Default to expandable panel style. This was made default on the settings side
in commit Ibd25bd1f058d49ab0560ed073c3b45782d165cbb
- Use a ContentObserver to monitor system settings.
- Moved static final variables out of Settings.java.
- Cleanup of a couple of untidy bits from initial jellybean merge.

Patch Set 2: Fix whitespace.

Change-Id: I738da63a4d59ad5ee52b1aa9e0e4f4c9c93a7576