How can you log into google oauth with a preset email and password?

Question

I'm working on a flutter app for a client.
This involves uploading files to a Google drive folder.

I've got it working with the Google sign in pop-up, but they want it so that the account used is separated from the device accounts.

Is this possible?

I don't think service accounts are right because they still want the email and password to be set in the app settings, and from what I understand service accounts need to be made and linked directly in the API console.

EDIT: here's my current code for logging in with the Google sign-in plugin;

 static final GoogleSignIn _googleSignIn = GoogleSignIn(
     scopes: [
       g.DriveApi.DriveScope,
       g.DriveApi.DriveMetadataReadonlyScope
     ]
   );
 static dynamic token;
 ...

static login() async{
    try {
      final GoogleSignInAccount googleUser = await _googleSignIn.signIn();
      final GoogleSignInAuthentication googleAuth = await googleUser
      .authentication;
      print("signed in " + googleAuth.accessToken);
      token = googleAuth.accessToken;
      _authHeaders = await _googleSignIn.currentUser.authHeaders;
    }catch(exception){
      //maybe offline
      print("login error: "+exception.toString());
    }
  }

So is it possible to use email and password instead of this method (which show's an account selection pop-up)?