Add SCREEN_BRIGHTNESS_FOR_VR to set of public settings.

This will allow the settings to be adjusted by any third party app,
just like SCREEN_BRIGHTNESS is today.

Additionally adds the default value to the SystemSettings database
on initial setup.

Test: After clean install check that the default value is in the DB via
  'adb shell settings get system screen_brightness_for_vr'
Test: Verify that SCREEN_BRIGHTNESS_FOR_VR can be changed by a 3rd party
  app with WRITE_SETTINGS permission.

Bug: 73090564
Change-Id: I93d167ab23652c39b0cf815eb40935be26b857a6
(cherry picked from commit 5956f49210f32b8ceabf860f904b5ca77ec9631c)