Animate taskbar background alpha and visibility alpha

Setup codepath to animate the Taskbar when going to and from Launcher,
primarily by listening for pause/resume signals but also hints from
gesture nav and AppToOverviewAnimationProvider.

Additionally, add TaskbarStateHandler to listen for Launcher state
changes if Taskbar is enabled. Combined, the end behavior is:

- Background alpha is 0 when Launcher is resumed, and 1 when Launcher
  is paused (we can make this animation more interesting later).
- Taskbar is always visible when Launcher is paused, otherwise its
  visibility is determined by multiple factors: LauncherState and
  whether the IME is showing.

Bug: 171917176
Change-Id: I7856fc979931c9d12d714dee11d179fd1b5a6968

quickstep/src/com/android/launcher3/BaseQuickstepLauncher.java

@@ -45,6 +45,7 @@ import com.android.launcher3.statehandlers.DepthController;
 import com.android.launcher3.statemanager.StateManager.StateHandler;
 import com.android.launcher3.taskbar.TaskbarContainerView;
 import com.android.launcher3.taskbar.TaskbarController;
+import com.android.launcher3.taskbar.TaskbarStateHandler;
 import com.android.launcher3.uioverrides.RecentsViewStateController;
 import com.android.launcher3.util.DisplayController;
 import com.android.launcher3.util.UiThreadHelper;
@@ -82,6 +83,7 @@ public abstract class BaseQuickstepLauncher extends Launcher
     private OverviewActionsView mActionsView;
 
     private @Nullable TaskbarController mTaskbarController;
+    private final TaskbarStateHandler mTaskbarStateHandler = new TaskbarStateHandler(this);
 
     @Override
     protected void onCreate(Bundle savedInstanceState) {
@@ -245,13 +247,23 @@ public abstract class BaseQuickstepLauncher extends Launcher
                 getWorkspace(),
                 getDepthController(),
                 new RecentsViewStateController(this),
-                new BackButtonAlphaHandler(this)};
+                new BackButtonAlphaHandler(this),
+                getTaskbarStateHandler(),
+        };
     }
 
     public DepthController getDepthController() {
         return mDepthController;
     }
 
+    public @Nullable TaskbarController getTaskbarController() {
+        return mTaskbarController;
+    }
+
+    public TaskbarStateHandler getTaskbarStateHandler() {
+        return mTaskbarStateHandler;
+    }
+
     @Override
     public void useFadeOutAnimationForLauncherStart(CancellationSignal signal) {
         QuickstepAppTransitionManagerImpl appTransitionManager =
@@ -296,6 +308,12 @@ public abstract class BaseQuickstepLauncher extends Launcher
             mDepthController.setActivityStarted(isStarted());
         }
 
+        if ((changeBits & ACTIVITY_STATE_RESUMED) != 0) {
+            if (mTaskbarController != null) {
+                mTaskbarController.onLauncherResumedOrPaused(hasBeenResumed());
+            }
+        }
+
         super.onActivityFlagsChanged(changeBits);
     }