Add managed app configuration profile

It would be really useful for the Nextcloud Android app to have a managed app configuration profile. This is so those of us that deploy the app through an EMM, can pre-configure the app so that the only thing the user has to do is enter their password.

The most obvious config field would be server address, but other things could be pre-set like a username variable pulled from the EMM. Other useful config settings would be the ability to lock down settings like sd card access, allow auto camera upload, allow only x number of accounts, sync frequency (which is not working properly at the moment), etc.

Google's documentation on managed configurations:
https://developer.android.com/work/managed-configurations