Invalidate taskview after setting obscure region

Otherwise, if nothing else about the taskview is changing, this
just sets a local property and doesn't apply it to the viewroot.

Bug: 382535017
Test: open a bubble (bar with handle). Then open handle menu. Then
      touch the menu.
Flag: EXEMPT bugfix
Change-Id: Icf2226856b4896b6c8d3e79e1e4347529ddeebd1