[auth] - Replicate web onAuthChanged() behavior by immediately calling the callback handler with the currentUser

Previously the callback handler would be called only when we have an auth state update, but the web api's
behavior is to call the handler once onAuthChanged() is attached. The initial value would then be undefined
if the user was never logged in.

Author: KkevinLi

src/app/auth/index.ts

@@ -44,11 +44,12 @@ export namespace auth {
     }
 
     // Completed will never be called, but it is here to closely follow the web api interface.
+    // When called, the callback handler will be passed in the currentUser or undefined if not signed in
     public onAuthStateChanged(handler: (user: User) => void,  error?: (err) => any, completed?: Unsubscribe): Unsubscribe {
       this.authStateChangedHandler = handler;
       if (error) this.authStateOnErrorHandler = error;
       console.log(">> added onAuthStateChanged handler");
-
+      handler(this.currentUser);
       return () => {
         this.authStateChangedHandler = undefined;
         this.authStateOnErrorHandler = undefined;