Don't call onModalnessUpdated if modalness stays the same

- Repeatedly calling onModalnessUpdated results in repeated calls of View.setAlpha, which caused regression in Overview loading time

Fix: 341266234
Test: android.platform.test.scenario.messenger.OpenAppFromQuickStepAndOpenQuickStepMicrobenchmark#testOpenQuickStepAndOpenApp
Flag: EXEMPT bugfix
Change-Id: Id756e80f20afafee2bb45e6a3ab861d83ab0b674

quickstep/src/com/android/quickstep/views/TaskView.kt

@@ -257,6 +257,9 @@ constructor(
      */
     protected var modalness = 0f
         set(value) {
+            if (field == value) {
+                return
+            }
             field = value
             onModalnessUpdated(field)
         }