Using Android Account Manager

I changed the RedditAccountManager from using a local SQL database to using the android account manager.

Using the RedditAccountManager got a bit more complicated, because you need to implement a callback, but now we can access android features like SyncAdapters or RestricedProfiles.

Proper error handling has yet to be included in each calling activity, maybe after the restructuring has finished.
As I am no UI Designer there might be room for improvements in the RedditAuthenticatorActivity ;)

I have done some testing with the old v1 MainActivity and fixed some bugs already, but extensive testing should be done after the restructuring.