Marks ScrimView unimportant for accessibility when Widgets screen is opened.

This prevents it from holding focus while the Widgets screen is visible
(after using Widgets action from the All Apps caret thingy).

Test: Manually followed steps provided in bug, and issue not seen after this change.
Fixes: 139918680
Change-Id: I280ac97fb7ff9fa67f1c6a1ce9cdfa9e451231eb