Fix TaskbarBackgroundRenderer not being applied correctly in 2 cases

- Rename isInAppAndNotStashed to areTaskbarIconsVisibleAndNotStashing,
  and check taskbarViewController.areIconsVisible() accordingly. This
  effectively handles the same as before, but also handles Overview,
  which is not inApp but does have areIconsVisible()
  - This fixes bubble scrim over overview taskbar, as well as showing
    taskbar background behind assistant in 3 button mode.
- Also fix nav bar button dark density changing when applying the
  separate background for assistant

Test: Open bubble and invoke assistant in overview, over IME, inside
an app, over home
Fixes: 266715337

Change-Id: Ie655de7abd1634e2165543d57664d7c7e054a986

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

@@ -349,10 +349,10 @@ public class TaskbarStashController implements TaskbarControllers.LoggableTaskba
 
 
     /**
-     * Returns whether the taskbar is currently visible and in an app.
+     * Returns whether the taskbar is currently visible and not in the process of being stashed.
      */
-    public boolean isInAppAndNotStashed() {
-        return !mIsStashed && isInApp();
+    public boolean isTaskbarVisibleAndNotStashing() {
+        return !mIsStashed && mControllers.taskbarViewController.areIconsVisible();
     }
 
     public boolean isInApp() {