Add unit tests to PresentationControllerTests

This change adds the new test case
testNewPresentationCannotShowOnPresentingDesk to
PresentationControllerTests to cover two additional scenarios
below:

- Multi-desk support (Icb244618be643dfc2989e18f05ae0770a709fa65)
 In the change, getTopDisplayFocusedRootTask() is replaced with
 getTopDisplayFocusedLeafTask() in PresentationController.

- Disallowing adding a new presentation to show over another
 presentation (I29ccb089aa057616138d039b3d62697a5ed63892)

(Why one test case for two scenarios? To test the first one, we
need to test one of the policies using Presentation#mHostTask(),
and the second one is a good example or that.)

Flag: com.android.window.flags.enable_presentation_for_connected_displays
Bug: 418850221
Test: PresentationTest
Change-Id: I296371fd4b3db01b76d58a5f148979c90e964a27