Add dim layer to PiP menu below buttons

Only show the dim when the menu buttons are shown to improve their
visibility.

Bug: 241744433
Test: manual - open a PiP and go to the menu. PiP content should get
darker. After pressing the back button or when entering the move menu
the dim should disappear.

Change-Id: I67256421994931467f2bc499bad5d5785213304a