Make bubble scrim be dark always

When the scrim is lighter & the pointer triangle is also
white, it's very hard to see and will not pass GAR.

Bug: 186441337
Test: atest ScrimControllerTest
Test: manual - expand bubble in dark & light theme on top of launcher
               and on top of some other app
             => notice scrim is always dark & pointer triangle is
                slightly more visible
Change-Id: I5e5c191bb813dc272284daf11e76c02ba17dc375