Support OAuth for Google (bitfireAT/davx5#289)

* Proof of concept (without auth state storage)
* Implement Bearer authentication, create network package
* Properly create/dispose AuthService
* Use proper ActivityResultContract
* Integrate into default login activity
* Change client ID to davx5integration@gmail.com
* Google Login: adapt login view
* Fix tests
* Don't allow empty Google account
* Move strings to resources