Yang auth

[butterflyforever]

This PR contains 2 updates

1. Integrated login feature using Auth API and Firebase Auth
2. Implemented Logout feature

The last commit:

1. Add login fragment. Separate login process from MainActivity
2. Reduce unnecessary variables in MainActivity
3. Using ViewModel to save the user
4. Fix bug in firebaseAuthWithGoogle addOnCompleteListener
5. Remove the activity in back stack.

[xyarco]

We can either bind the viewmodel's currentUser email to fragment_me.xml or update this in the observer when currentUser is not null (maybe move the observer down here).

[xyarco]

Why do we want to wrap the Firebaseuser to antoher class. Seems we can just have something like this

val currentUser : MutableLiveData<FirebaseUser?> by lazy {
    MutableLiveData<FirebaseUser?> (FirebaseAuth.getInstance().currentUser)
}

And we can just determine the isLogin status by checking the value of currentUser. Keeping another isLogin might be error prone, cuz you will need to update 2 values together if something has changed. What if you forget to update one of them, it can cause the 2 fields out of sync.

[xyarco]

These seems unnecessary. In the fragments, we can just observe the currentUser, something like this

signInVM.currentUser.observe(this, Observer {
  if (it != null) {
    // do something with user logged in
  } else {
    // do something with user not logged in
  }
})

[xyarco]

Suggested change

	private fun googleSignInInit() {
	val gso = GoogleSignInOptions.Builder(GoogleSignInOptions.DEFAULT_SIGN_IN)
	.requestIdToken(context.getString(R.string.default_web_client_id))
	.requestEmail()
	.build()
	googleSignInClient = GoogleSignIn.getClient(activity, gso)
	Log.d(SIGN_IN_VM_TAG, "googleSignInClientInit")
	}
	private fun getGoogleSignInClient() : GoogleSignInClient {
	val gso = GoogleSignInOptions.Builder(GoogleSignInOptions.DEFAULT_SIGN_IN)
	.requestIdToken(context.getString(R.string.default_web_client_id))
	.requestEmail()
	.build()
	Log.d(SIGN_IN_VM_TAG, "googleSignInClientInit")
	return GoogleSignIn.getClient(activity, gso)
	}

Seems this can just return a GoogleSignInClient. And the googleSignInClient field on top is not necessary.