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
diff --git a/libs/WindowManager/Shell/Android.bp b/libs/WindowManager/Shell/Android.bp
index f615ad6..0f45219 100644
--- a/libs/WindowManager/Shell/Android.bp
+++ b/libs/WindowManager/Shell/Android.bp
@@ -47,6 +47,7 @@
         "src/com/android/wm/shell/sysui/ShellSharedConstants.java",
         "src/com/android/wm/shell/common/TransactionPool.java",
         "src/com/android/wm/shell/animation/Interpolators.java",
+        "src/com/android/wm/shell/pip/PipContentOverlay.java",
         "src/com/android/wm/shell/startingsurface/SplashScreenExitAnimationUtils.java",
     ],
     path: "src",