UdfpsHelper: Track brightness mirror to hide/show dim layer

This provides a much deeper integration with UDFPS dim layer and the new
brightness slider such that we don't need to look for APIs that can
track temporary brightness. That is because when the brightness mirror
is shown, there is no necessity for the UDFPS dim layer to be present.
This change also isn't functional with the legacy slider.

It also fixes the issue where the dim layer is hidden with QS expansion
-> Device Control where the alternate bouncer is brought up.

Test: Ensure that appropriate transparency of the dim layer is set when
dragging or selecting the preferred brightness with the slider.

Change-Id: I53f78cbcfd3b42a3b5dbe6110424687ebfac102d