Dismiss UserSwitchingDialog on UiHandler thread.

This CL makes sure the dismissal is always done on the UI thread to
prevent CalledFromWrongThreadException.

Bug: 284085833
Test: atest UserControllerTest
Change-Id: I7c0b35be2a171648c20b50cee6dea78ba7a2cf5b

services/core/java/com/android/server/am/UserController.java

@@ -1972,7 +1972,7 @@ class UserController implements Handler.Callback {
     }
 
     private void dismissUserSwitchDialog(Runnable onDismissed) {
-        mInjector.dismissUserSwitchingDialog(onDismissed);
+        mUiHandler.post(() -> mInjector.dismissUserSwitchingDialog(onDismissed));
     }
 
     private void showUserSwitchDialog(Pair<UserInfo, UserInfo> fromToUserPair) {

services/tests/servicestests/src/com/android/server/am/UserControllerTest.java

@@ -1342,6 +1342,10 @@ public class UserControllerTest {
             Message copy = new Message();
             copy.copyFrom(msg);
             mMessages.add(copy);
+            if (msg.getCallback() != null) {
+                msg.getCallback().run();
+                msg.setCallback(null);
+            }
             return super.sendMessageAtTime(msg, uptimeMillis);
         }
     }