Dismiss PiP to split screen if applicable

When splt screen is present, dismiss PiP should result in
split-secondary instead of fullscreen mode.

Fixed also in this CL that when a PiP window transitions to split
screen, a picture-in-picture mode change event should be dispatched to
client.

Video: http://go/recall/-/aaaaaabFQoRHlzixHdtY/dP7U1AtIBXPvLwMp7fFXN

Bug: 154284314
Test: atest PinnedStackTests
Test: manually dismiss PiP when split screen is present
Change-Id: I99b33c1be37d13572e540d7d30b96ed75d00323d