Skip updating taskbar background alpha in phone mode

Based on https://b.corp.google.com/issues/361593564#comment4

Bug: 361593564
Test: manual
Flag: EXEMPT bugfix
Change-Id: I1b23847af707e6fa208345b87953b53f79543bfe

quickstep/src/com/android/launcher3/taskbar/TaskbarDragLayerController.java

@@ -171,6 +171,10 @@ public class TaskbarDragLayerController implements TaskbarControllers.LoggableTa
     }
 
     private void updateBackgroundAlpha() {
+        if (mActivity.isPhoneMode()) {
+            return;
+        }
+
         final float bgNavbar = mBgNavbar.value;
         final float bgTaskbar = mBgTaskbar.value * mKeyguardBgTaskbar.value
                 * mNotificationShadeBgTaskbar.value * mImeBgTaskbar.value