Fix the bubble expanded scrim colour to use the shared constant

Flag: com.android.wm.shell.enable_bubble_bar
Test: manual - most noticeable in light theme with persistent taskbar
               so set to that and then expand bubbles and observe
               that the scrim on taskbar isn't darker than the scrim
               on the rest of the screen
Bug: 376141576
Change-Id: I386525d48724139239782871eb7f6b4bf469cb92