Check only visible windows when looking for occluding activities

Consider only the currently visible activities when looking for opaque
ones, for the sake of deciding which dim to use.

AnnotateActivity is a fullscreen opaque activity in the same task as ChooserActivity. As Annotate opened on top of Chooser, the task became opaque without waiting for Annotate to be visible, This was causing the dim to fade out too soon.

Fix: 281632483
Test: DimmerTests
Change-Id: Idc8c3192d68eee25d4221c92909deacdca85f6c3