Question: Drop use of androids build in AccountManager

@tobiasKaminsky Just throwing in some thoughts here. As we had a bunch of issues already when trying to figure out which accounts exist, I was thinking about if we can drop the need to use the Android "AccountManager" at all. As of Android 9 getting accounts from another app is even more difficult. Therefore dropping it makes things easier for all involved parties.

Advantages:

• reauthentication works without the need to request the account permission again (way easier for client apps as we don't need to restart the login flow)
• Custom dialog for choosing the account (nextcloud design / theme)
• reduce complexity of account handling (in the sso library)

Disadvantages:

• None?

So basically what I'm proposing is:

• Instead of calling the account manager to show us available accounts, we create a new custom activity in the files app for that. (Reference)
• The files app lists all accounts available (as the files app has the required permission for that)
• The users selects an account here

This also might be interesting for @desperateCoder @stefan-niedermann

Start with the referenced AccountImporter.java in the Android-SingleSignOn library and review how AccountManager currently supplies accounts. Then inspect the proposed files-app integration and custom activity flow. Done means the files app can list available accounts and return the user's selection without requiring the client to use AccountManager.