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

libs/WindowManager/Shell/Android.bp

@@ -47,6 +47,7 @@ filegroup {
         "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",

libs/WindowManager/Shell/src/com/android/wm/shell/pip/PipContentOverlay.java

@@ -16,6 +16,7 @@
 
 package com.android.wm.shell.pip;
 
+import android.annotation.Nullable;
 import android.content.Context;
 import android.content.res.TypedArray;
 import android.graphics.Color;
@@ -41,6 +42,11 @@ public abstract class PipContentOverlay {
         }
     }
 
+    @Nullable
+    public SurfaceControl getLeash() {
+        return mLeash;
+    }
+
     /**
      * Animates the internal {@link #mLeash} by a given fraction.
      * @param atomicTx {@link SurfaceControl.Transaction} to operate, you should not explicitly