Ensure at most one pip-dismiss-overlay

PipDismissTargetHandler#init can be called multiple times, whenever a
new dismiss view (and its container) is created, remove the old one from
view hierarchy first.

Bug: 207495181
Test: manual, change wallpaper multiple times to trigger multiple init
      calls in PipDismissTargetHandler and ensures there is at most one
      pip-dismiss-overlay in the view hierarchy.
Change-Id: I4a0e058d1d002a6420974f41b94b70316d3444a3

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

@@ -120,6 +120,11 @@ public class PipDismissTargetHandler implements ViewTreeObserver.OnPreDrawListen
         mEnableDismissDragToEdge = res.getBoolean(R.bool.config_pipEnableDismissDragToEdge);
         mDismissAreaHeight = res.getDimensionPixelSize(R.dimen.floating_dismiss_gradient_height);
 
+        if (mTargetViewContainer != null) {
+            // init can be called multiple times, remove the old one from view hierarchy first.
+            mWindowManager.removeViewImmediate(mTargetViewContainer);
+        }
+
         mTargetView = new DismissCircleView(mContext);
         mTargetViewContainer = new FrameLayout(mContext);
         mTargetViewContainer.setBackgroundDrawable(