You can't directly access the database from anything that connects to a website - the database is on the server.
If you've rewritten it to run completely in an Android device, you'll have to use SQLite to maintain a database on the device. If you need a public database (everyone using the app has to have access to the database), you'll have to write an API on the site, taking in the data needed and outputting the data needed, and the app will have to access the site's API.
You can't write the Android app in Drupal, so I assume you mean that the site uses Drupal. Since Drupal
does access the database, you have your choice of using Drupal or the underlying PHP to access the database. (If you use PHP,
PLEASE use PDO prepared statements to avoid SQL injection by hackers. It's too easy, in using non-prepared statements, for a hacker to just dump your entire database. Or, if he's malignant enough, to drop all your tables, leaving you with no database and no means of recovery [see
https://www.explainxkcd.com/wiki/index.php/327:_Exploits_of_a_Mom for an explanation of "Little Bobby Tables", a common bug that inexperienced PHP programmers write])