PiP: Disable enter-split button when not visible.

Bug: 209360831
Test: Enter PiP, press on where split-button should be - doesn't do
anything

Change-Id: I456cf7f3aa2aecdf3dae10d968c28d92e985e7dd

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

@@ -189,7 +189,7 @@ public class PipMenuView extends FrameLayout {
         mEnterSplitButton = findViewById(R.id.enter_split);
         mEnterSplitButton.setAlpha(0);
         mEnterSplitButton.setOnClickListener(v -> {
-            if (mMenuContainer.getAlpha() != 0) {
+            if (mEnterSplitButton.getAlpha() != 0) {
                 enterSplit();
             }
         });