Keep the task in main stage at top after dismissed split due to fold

Fix: 189333036
Fix: 189723946
Bug: 188010345
Test: atest WMShellUnitTests
Test: trigger staged split, the task in main stage will remain at top
after dismissed split due to fold.

Change-Id: If2a795bed7073efcea70129e62aacdaad3efa602

libs/WindowManager/Shell/src/com/android/wm/shell/splitscreen/StageCoordinator.java

@@ -609,7 +609,7 @@ class StageCoordinator implements SplitLayout.SplitLayoutHandler,
 
     private void onFoldedStateChanged(boolean folded) {
         if (folded && mMainStage.isActive()) {
-            exitSplitScreen();
+            exitSplitScreen(mMainStage);
         }
     }