Get rid of #getViewTreeObserver

Which cause app crash by an null exception

Test: Manually
Fixes: 156105622
Change-Id: Ife0d59c108e693e4603e6bd3e39d2746fb479306
diff --git a/src/com/android/customization/picker/CustomizationPickerActivity.java b/src/com/android/customization/picker/CustomizationPickerActivity.java
index cf7c350..902b299 100644
--- a/src/com/android/customization/picker/CustomizationPickerActivity.java
+++ b/src/com/android/customization/picker/CustomizationPickerActivity.java
@@ -145,13 +145,8 @@
         }
 
         mBottomActionBar = findViewById(R.id.bottom_actionbar);
-        mBottomActionBar.getViewTreeObserver().addOnGlobalLayoutListener(() -> {
-            // Only update the visibility of mBottomNav when mBottomActionBar visibility changes.
-            // Since the listener will be triggered by mBottomActionBar and its child views.
-            if (mBottomActionBar.getVisibility() == mBottomNav.getVisibility()) {
-                mBottomNav.setVisibility(mBottomActionBar.isVisible() ? View.GONE : View.VISIBLE);
-            }
-        });
+        mBottomActionBar.addVisibilityChangeListener(
+                isVisible -> mBottomNav.setVisibility(isVisible ? View.GONE : View.VISIBLE));
     }
 
     @Override