Deactivate touch target for splitscreen button

Remove ripple after touching the splitscreen button
(with id of android:id="@+id/enter_split" defined in pip_menu.xml)
by setting the ImageButton setEnabled to true in PipMenuView.java

Test: atest SystemUITests
Test: manually, reproduced steps in the bug listed
Bug: 239613989
Change-Id: I105966feb22a0ccc5f11b59848c81f9ba682c3eb

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

@@ -207,6 +207,9 @@ public class PipMenuView extends FrameLayout {
             }
         });
 
+        // this disables the ripples
+        mEnterSplitButton.setEnabled(false);
+
         findViewById(R.id.resize_handle).setAlpha(0);
 
         mActionsGroup = findViewById(R.id.actions_group);