Move listening to brightness changes to onStart/onStop.

Currently, we register the brightness listener in onResume() and stop
listening in onPause(). When we launch the brightness selection bar, the
display settings is paused, and hence we don't get the brightness
update, and only refresh the percentage when the brightness bar is
dismissed.

Moving the listening logic to onStart()/onStop() instead, so that we
will get the updates all the time when the display settings is launched.

Change-Id: I6ed9c8485fdf7721374baff5cea5f7011b63d7ea
Fixes: 65981061
Test: make RunSettingsRoboTests