Initial TaskbarUnitTestRule with example overlay controller tests.

Flag: TEST_ONLY
Bug: 230027385
Test: TaskbarOverlayControllerTest
Change-Id: I858906ece7e67677962ec8b4432bfcca5ec30283

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

@@ -1605,4 +1605,9 @@ public class TaskbarActivityContext extends BaseTaskbarContext {
     boolean canToggleHomeAllApps() {
         return mControllers.uiController.canToggleHomeAllApps();
     }
+
+    @VisibleForTesting
+    public TaskbarControllers getControllers() {
+        return mControllers;
+    }
 }