Allow Launcher to use PipContentOverlay

- Export PipContentOverlay to library shared with Launcher
- Add PipContentOverlay#getLeash, which Launch propagates for cleanup

This refactor makes it easier to experiment the app icon overlay (solid
background with app icon as an overlay).

Bug: 265998256
Test: Enter PiP from Google Maps, ensure the content overlay works
Change-Id: If011c650e237c5834556e3b250200f63f78f84e2