Test for skipping split screen check in observer

BubblesTransitionObserver can skip checking the transition if there are
no bubbles or if the transition is not an open type.
Add a test for this.

Bug: 418591790
Test: atest WMShellUnitTests:BubblesTransitionObserverTest
Flag: com.android.wm.shell.enable_create_any_bubble
Change-Id: I1d78a4a8ee858cf469b5b9b20b402e4986897e84