Implement pinch resizing in PiP2

Use transitions and PipTransitionState to allow
pinch-to-resize gestures in PiP2.

At the moment, the animator for when both pointers
are release is not set up so after pinching is complete
and pointers are release a transition is played w/o
any animation - just a jumpcut instead.

Bug: 340883807
Test: enter PiP2 and pinch-to-resize
Change-Id: Ic2a11444506663c96a2c8ac052092e84ea292889