Ensure dismiss button within closePipWindow

This is to comply with ag/15351291 that the dismiss button is only
available after the tap-to-reveal PiP menu animation is completed.

Bug: 176822698
Test: atest WMShellFlickerTests:ExitPipWithDismissButtonTest
Change-Id: Iae0eae56b0fb358e3d39454de59c8ba64e65ee7b