Handling user presence in firebase

Question

i want to make users appear online in my app when they are in any activity until they close or minimize the app. below is what i did in MainActivity.java

  if (mAuth.getCurrentUser() != null) {
        mUserRef = FirebaseDatabase.getInstance().getReference().child("Users").child(mAuth.getCurrentUser().getUid());

    }
}


    @Override
    public void onStart () {
        super.onStart();
        // Check if user is signed in (non-null) and update UI accordingly.
        FirebaseUser currentUser = mAuth.getCurrentUser();

        if (currentUser == null) {
            sendtostart();
        } else {
            mUserRef.child("online").setValue("true");
        }
    }

    @Override
    protected void onStop () {
        super.onStop();

        FirebaseUser currentUser = mAuth.getCurrentUser();

        if (currentUser != null) {
            mUserRef.child("online").setValue(ServerValue.TIMESTAMP);
        }
    }

    private void sendtostart () {
        Intent startIntent = new Intent(MainActivity.this, StartActivity.class);
        startActivity(startIntent);
        finish();
    }

but this only works when user is in the main activity so when a user switched to Chat Activity or Profile Activity they appear offline. below is what i tried in my ChatActivity.java but it did not work

    if (mAuth.getCurrentUser() != null) {
        mUserRef = FirebaseDatabase.getInstance().getReference().child("Users").child(mAuth.getCurrentUser().getUid());

    }

}
@Override
public void onStart() {
    super.onStart();

    FirebaseUser currentUser = mAuth.getCurrentUser();

    if (currentUser != null){
        mUserRef.child("online").setValue("true");

    }else {
        mUserRef.child("online").setValue("false");

    }