Verify transition from trampoline to bubble

Check that we first have a layer for the trampoline and then the
launched app.

Bug: 439498605
Flag: EXEMPT TEST_ONLY
Test: atest WMShellExplicitFlickerTestsBubbles:EnterBubbleFromHomeScreenTrampolineTest
Change-Id: I2272a84577e579f5f505a5d17498f0ae79d856c1