Simplified ActivityStack.shouldBeVisible()

The method can now be simplified since we have a stack per task so
if a stack should be visible is now a question of if it is occluded
by another stack.

Also,
- Fixed an issue where the windowing mode of the primary split-screen
stack was changing to split-screen-secondary instead of fullscreen
when we are exiting split-screen mode because we are not allowed to
to create fullscreen stack when there is a primary split-screen stack. We
now clear the reference to the primary split-screen stack when exiting
split-screen mode.
- Re-worked windowing mode resolution to be inside ActivityDisplay
object since the determination of the windowing mode is dependant on the
display.

Test: bit FrameworksServicesTests:com.android.server.am.ActivityStackTests
Test: Existing tests pass.
Test: go/wm-smoke
Bug: 64146578
Fixes: 67914671
Change-Id: I7e8cfe49fbf6a5836ded022bb11adcde58ae689c