Allow multiple tasks to change in one transition

Not sure why I added this restriction. It breaks split-screen
transitions since enter/exit involves both splits changing
mode.

Bug: 151881448
Test: existing tests pass, transitions with split-screen (when
      enabled) work properly.
Change-Id: I176ee20cfc288ba2275a0739fcc0059435a0337f

services/core/java/com/android/server/wm/Task.java

@@ -2477,7 +2477,6 @@ class Task extends WindowContainer<WindowContainer> {
     private boolean shouldStartChangeTransition(int prevWinMode, int newWinMode) {
         if (mWmService.mDisableTransitionAnimation
                 || !isVisible()
-                || getDisplayContent().mAppTransition.isTransitionSet()
                 || getSurfaceControl() == null
                 || !isLeafTask()) {
             return false;