Implement PIP Leash resizing.

This CL does a couple of things:
- Break apart resizePinnedStack into animateResizeStack and
resizeStackIntermediate, in which the latter is expected to only do
maniuplation on the leash itself but don't send any
WindowContainerTransaction until #finishResize is called.
- Break apart the coupling where the duration signifies whether we only
do manipulation on the leash, or send over a WCT. Now we can choose to
either resize leash only, or animate correctly an entire resize with
animation = 0.
- Have PipResizeGestureHandler call on #finishResize and
resizePipStackIntermediate during MOVE and UP events, respectively

Bug: 147361175
Test: Manually. Resize PIP.
Change-Id: Id3953904a7d626880855eab043abfb8d8a8d9b11