Workaround issue with pip overlay not hiding cleanly.

- We were just finishing the activity which was causing issues with the
  wrong app transition being applied.  We actually can just hide the menu
  activity first before finishing it like we do elsewhere.

Bug: 38222481
Test: Open PiP, launch to show assistant

Change-Id: I348f8a17f76006bcef46fe01c87deae0b89f6586

packages/SystemUI/src/com/android/systemui/pip/phone/PipMenuActivity.java

@@ -221,9 +221,9 @@ public class PipMenuActivity extends Activity {
     protected void onUserLeaveHint() {
         super.onUserLeaveHint();
 
-        // If another task is starting on top of the menu, then finish it so that it can be
+        // If another task is starting on top of the menu, then hide and finish it so that it can be
         // recreated on the top next time it starts
-        finish();
+        hideMenu();
     }
 
     @Override