Add Brightness setting for VR Mode.

This change saves and loads a different brightness setting when the user
goes in and out of VR Mode.

Test: runtests frameworks-core
Test: Turn vr mode on and change brightness (should remain separate from
normal brightness)

Bug: 30984614
Change-Id: If3c3e81b592e0c6fd037e5783559683e5cb58379
17 files changed