[PiP on Desktop] Fix drag-to-desktop freezing when PiP is active

Recall: http://recall/clips/7ab57d60-a1b9-40ee-a2f7-1f632e22c486

Drag-to-desktop gesture starts two transitions, and when PiP is active,
a RESIZE_PIP transition is started when the drag gesture starts due to
KCA updates. The Pip transition comes in between the two desktop
transitions and blocks the second one from merging, causing the
transitions to not be finished.

This CL blocks a RESIZE_PIP transition from starting if there is ongoing
drag-to-desktop gesture. Further work will be done in a future CL to
trigger KCA updates once we are in the Desktop session.

Bug: 397765735
Test: atest PipDesktopStateTest PipTransitionStateTest
Flag: com.android.window.flags.enable_desktop_windowing_pip

Change-Id: I7a41cd0298e7e0cf1b58c9b8b2495b88c9f7ed6a