Sign out

private void signOutTestuser() {
    FirebaseUser user = mAuth.getCurrentUser();
    if (user == null) {
         Toast.makeText(getApplicationContext(), "No user signed in.",
         Toast.LENGTH_SHORT).show();
         return;
        }
    FirebaseAuth.getInstance().signOut();
    Toast.makeText(MainActivity.this, "You are signed out.",
    Toast.LENGTH_SHORT).show();
}

Note that signOut() returns immediately without using a Task.