I am making a simple authentication app in Android using Firebase authentication. Till now I am successful in signing the user in, however the issue is that the user remains
You can simply call this
FirebaseAuth.getInstance().signOut();
If you want to perform some action after sign out then use this one.
public void onClick(View v) {
    if (v.getId() == R.id.sign_out) {
        AuthUI.getInstance()
            .signOut(this)
            .addOnCompleteListener(new OnCompleteListener() {
                public void onComplete(@NonNull Task task) {
                    // user is now signed out
                    startActivity(new Intent(MyActivity.this, SignInActivity.class));
                    finish();
                }
            });
        }
    }