Add support for multiple accounts with the same cloud provider

[bloatmode]

Basic Info

I'm using Android in version: 9.0

I'm running Cryptomator in version: 1.4.2

Description

I have two different OneDrive accounts, one is my personal one and the other was given to me by my University. It would be nice to be able to log into both and open the vaults I have in both accounts. I was able to do this with the official OneDrive app and with FX file explorer, so I imagine it would be possible to implement it in your app.

[bloatmode]

When I opened this issue Cryptomator for Android wasn't open source, but now one of us could implement the feature. I don't have much free time at the moment so I wouldn't be able to do it, maybe next year.

[SailReal]

Multiple accounts for OneDrive will be available with the next update, see #405 (comment), but for Google Drive and Dropbox this still needs to be implemented.