Add double tap callback for divider bar

Add a callback in split layout to notify registered listener when users
double tapped the divider bar. Use this callback to switch side stage
position.

Bug: 176061049
Test: atest SplitLayoutTests
Test: manual check stage position changed when double tapped divider bar
Change-Id: Id3929b01a194b8eb0838cc892156bbbe48dc8690

libs/WindowManager/Shell/tests/unittest/src/com/android/wm/shell/common/split/SplitLayoutTests.java

@@ -82,6 +82,12 @@ public class SplitLayoutTests extends ShellTestCase {
         verify(mLayoutChangeListener).onBoundsChanged(any(SplitLayout.class));
     }
 
+    @Test
+    public void testOnDoubleTappedDivider() {
+        mSplitLayout.onDoubleTappedDivider();
+        verify(mLayoutChangeListener).onDoubleTappedDivider();
+    }
+
     @Test
     @UiThreadTest
     public void testSnapToDismissTarget() {