Add logs for Brightness changes

Also, use a StateFlow in the ViewModel so the last value of the brightness is
kept when there are no subscribers.

Fixes: 331199596
Fixes: 339262873
Test: dump and bugreport
Flag: com.android.systemui.qs_ui_refactor
Change-Id: Ie7198cef8e93e6aed064956a10ac2d22b98e4daf