PiP: support direct entering split from PiP.

Add a new "enter split" button that allows user to go directly into
split screen on large screen devices from PiP.

- Add button
- Calculate split-screen location based on current PiP bounds
- Merge all WCT into one after animating

Video: http://recall/-/gu5DjQFZPMFpwgaXdl4cV3/gnNkSpC72tlTGP2mrKR07R

Bug: 202740657
Test: Enter PIP, open another app, tap "enter split"

Change-Id: I0829c910b2873ed4c4b0ea4fe0ca28b7f87736d8