Make VR brightness adjustable again.

After some code adjustment, changes to VR's brightness setting
were not respected.  Add code to listen for the changes and clamping
code to support the min & max values for VR brightness.

Bug: 73090564

Test: while in VR mode, verify that the following command updates
brightness: 'adb shell system put screen_brightness_for_vr
<VALUE>'

Change-Id: I1ce411c5b5fff11c40d1a6ac0b20588b69d2e64e