Add FallbackTaskbarUIController

- Refactor TaskbarManager to accept StatefulActivity instead of BaseQuickstepLauncher
- Update the new StatefulActivity in onOverviewTargetChange()
- Cleanup LauncherTaskbarUIController a bit to avoid initializing TaskbarActivityContext, TaskbarDragLayer, and TaskbarViewController in the constructor, and instead get these from mControllers after init()

Test: No change in behavior
Bug: 188790554
Change-Id: Ic4d69d73deb6dada416876eb265db57803c3adfd

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

@@ -0,0 +1,30 @@
+/*
+ * Copyright (C) 2021 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.android.launcher3.taskbar;
+
+import com.android.quickstep.RecentsActivity;
+
+/**
+ * A data source which integrates with the fallback RecentsActivity instance (for 3P launchers).
+ */
+public class FallbackTaskbarUIController extends TaskbarUIController {
+
+    private final RecentsActivity mRecentsActivity;
+
+    public FallbackTaskbarUIController(RecentsActivity recentsActivity) {
+        mRecentsActivity = recentsActivity;
+    }
+}