Add trace logs for UserController#dispatchUserSwitchComplete.

To track how long it takes for each mUserSwitchObservers to process
IUserSwitchObserver#onUserSwitchComplete call and when that happens.

Bug: 239497256
Test: atest MultiUserPerfTests:android.multiuser.UserLifecycleTests#switchUser_stopped --no-bazel-mode
Change-Id: I9c618b6bb2247d93f624a8f9b9351608d4579000

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

@@ -1872,15 +1872,21 @@ class UserController implements Handler.Callback {
     /** Called on handler thread */
     @VisibleForTesting
     void dispatchUserSwitchComplete(@UserIdInt int userId) {
+        final TimingsTraceAndSlog t = new TimingsTraceAndSlog();
+        t.traceBegin("dispatchUserSwitchComplete-" + userId);
         mInjector.getWindowManager().setSwitchingUser(false);
         final int observerCount = mUserSwitchObservers.beginBroadcast();
         for (int i = 0; i < observerCount; i++) {
             try {
+                t.traceBegin("onUserSwitchComplete-" + userId + " #" + i + " "
+                        + mUserSwitchObservers.getBroadcastCookie(i));
                 mUserSwitchObservers.getBroadcastItem(i).onUserSwitchComplete(userId);
+                t.traceEnd();
             } catch (RemoteException e) {
             }
         }
         mUserSwitchObservers.finishBroadcast();
+        t.traceEnd();
     }
 
     private void dispatchLockedBootComplete(@UserIdInt int userId) {