Ensure we use the fullscreen stack bounds when expanding PiP.

- When in split screen, the PiP needs to animate to the fullscreen stack
  bounds when expanding.
- Created PinnedStackWindowController to house pinned-stack specific
  logic.

Bug: 35396882
Test: Expand PiP activity while split

Change-Id: If7397843743ce11b676f5566eba90e3442289fec
Signed-off-by: Winson Chung <winsonc@google.com>