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();
     }