build an android application that send an email via gmail and uses oauth2

I need to write an android application that send an email via gmail and uses Oauth2 for authtincation. The application need to store and manage the token persistently. The token need to be stored on the mobile either as text file or in embded database. In addition the application need to use gradle 7 and above to manage the builds.
The application need to be modular and function need to be modular in case of any upgrade later.