Re-implement pinch-to-resize gesture.

This removes ScaleGestureDetector and instead just do the calculation
inside the class itself. As user sets two input points on the PiP
surface, this signals the start of the gesture, and as user drags the
two fingers, it resizes the PiP to the maximum possible size. After the
user releases their fingers, then it auto-offsets/snaps to the right
position.

Bug: 166478885
Test: Start a pinch-to-resize while feature flag is on
Change-Id: I634bd14e97f0cf0cc7bf9c1a1b64a1f12130e0e9