Share the scrim value with launcher

When I changed the scrim in ag/24343831 I didn't do it in taskbar as
well, extract the value into common class so it can be shared with
launcher in one place.

Test: manual - expand a bubble and check the scrim
Bug: 294396107
Change-Id: I810889ee81b70c90f1bfad646cb4c6854f6d7a01