Fix bug where Taskbar was not auto-hiding after splitscreen launch from icon menu

This patch fixes a bug where the transient Taskbar was not hiding properly after splitscreen was initiated. When the user is inside an app and launches splitscreen by longpressing on a Taskbar icon and selecting the split button, the transient Taskbar should hide right away. This is an equivalent action to dragging the Taskbar icon up to create a split, and should hide the Taskbar so that other UI elements (like system-level toasts and error messages) can be seen.

The bug occurred because updateAndAnimateTransientTaskbar() is not being called in this specific code path to stash the Taskbar.

Fixed by adding a new call to updateAndAnimateTransientTaskbar upon clicking the splitscreen menu button.

Fixes: 272292897
Test: Manual
Change-Id: I64a9acfc41ddcaba4d9f43eb216458de44b4c9a4

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

@@ -65,4 +65,10 @@ public abstract class BaseTaskbarContext extends ContextThemeWrapper implements
 
     /** Callback invoked when a popup is shown or closed within this context. */
     public abstract void onPopupVisibilityChanged(boolean isVisible);
+
+    /**
+     * Callback invoked when user attempts to split the screen through a long-press menu in Taskbar
+     * or AllApps.
+     */
+    public abstract void onSplitScreenMenuButtonClicked();
 }