AOD: Add support for dimming in software

Some devices cannot dim low enough in hardware in low power states.
To work around that, keep the front scrim at higher opacity when needed
to simulate lower brightness.

Bug: 63995944
Test: Configure scrim in config.xml; use device in very dim light at night, observe AOD is sufficiently dim.
Change-Id: I92fffedeea89b9327d76dfb184c5c761364ead11