A few things:
- Code is a bit messy with a lot of commented boilerplate mess, which you can remove anyway.
- Make sure you use appropriate signing key and parameters (like 'App name' in GoogleDriveFragment.java) with respect to your google developer console configuration.
- I might clean code and document things once my free time is not dedicated to finding new job :)
As always, Pull requests are welcome! Stay Safe people, have a nice day!