Add method to determine if launcher is in split selection state

* Available for both launcher and taskbar via
ActivityContext

Bug: 295981634
Change-Id: I8767deea0b2432da3047209037874c966b83c3b3

quickstep/src/com/android/launcher3/uioverrides/QuickstepLauncher.java

@@ -691,7 +691,7 @@ public class QuickstepLauncher extends Launcher {
     }
 
     @Override
-    protected boolean isSplitSelectionEnabled() {
+    public boolean isSplitSelectionEnabled() {
         return mSplitSelectStateController.isSplitSelectActive();
     }
 

src/com/android/launcher3/Launcher.java

@@ -3103,7 +3103,7 @@ public class Launcher extends StatefulActivity<LauncherState>
     }
 
     /** To be overridden by subclasses */
-    protected boolean isSplitSelectionEnabled() {
+    public boolean isSplitSelectionEnabled() {
         // Overridden
         return false;
     }

src/com/android/launcher3/views/ActivityContext.java

@@ -145,6 +145,15 @@ public interface ActivityContext {
         // Overridden, intentionally empty
     }
 
+    /**
+     * @return {@code true} if user has selected the first split app and is in the process of
+     *         selecting the second
+     */
+    default boolean isSplitSelectionEnabled() {
+        // Overridden
+        return false;
+    }
+
     /**
      * The root view to support drag-and-drop and popup support.
      */