Adds the FloatingContentCoordinator.

This class helps PIP, Bubbles, and future additions to the floating family peacefully co-exist.

Design doc: go/pip-bubbles-unification#heading=h.ip7f8yd5i199

This is the initial CL - a followup will add the permanent/temporary move logic, which restores floating content's original positions if a move is aborted.

Bug: 138115889
Test: atest SystemUITests
Change-Id: I762ce25700523ff581e1dc827de3bb9128ea97a2