[Desktop Mode] Show indicators under Taskbar app icons for running apps

In Desktop Mode we show running Desktop apps in the Taskbar. With this
change we also show an indicator under the app icon for each such
running app.

Bug: 332504528
Test: manual
Test: DesktopTaskbarRunningAppsControllerTest
Flag: ACONFIG com.android.window.flags.Flags.enableDesktopWindowingTaskbarRunningApps DEVELOPMENT
Change-Id: If0906dab8ad0bd8a78d93a4e99db47550e763bed

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

@@ -15,6 +15,8 @@
  */
 package com.android.launcher3.taskbar;
 
+import static java.util.Collections.emptySet;
+
 import android.util.SparseArray;
 
 import androidx.annotation.CallSuper;
@@ -22,6 +24,8 @@ import androidx.annotation.CallSuper;
 import com.android.launcher3.model.data.AppInfo;
 import com.android.launcher3.model.data.ItemInfo;
 
+import java.util.Set;
+
 /**
  * Base class for providing recent apps functionality
  */
@@ -43,7 +47,8 @@ public class TaskbarRecentAppsController {
     }
 
     /** Stores the current {@link AppInfo} instances, no-op except in desktop environment. */
-    protected void setApps(AppInfo[] apps) { }
+    protected void setApps(AppInfo[] apps) {
+    }
 
     /**
      * Indicates whether recent apps functionality is enabled, should return false except in
@@ -59,5 +64,11 @@ public class TaskbarRecentAppsController {
     }
 
     /** Called to update the list of currently running apps, no-op except in desktop environment. */
-    protected void updateRunningApps(SparseArray<ItemInfo> hotseatItems) { }
+    protected void updateRunningApps(SparseArray<ItemInfo> hotseatItems) {
+    }
+
+    /** Returns the currently running apps, or an empty Set if outside of Desktop environment. */
+    public Set<String> getRunningApps() {
+        return emptySet();
+    }
 }