Removes the linked account verification from LinkDropboxFragment

It now assumes that when the LinkDropboxFragment is displayed there's no already linked account.
The MainActivity is going to manage this.
Adds the linked Dropbox account verification to MainActivity

This way the MainActivity decides when the LinkDropboxFragment is showed or not.
Removes the DropboxAuthManager param from the onDropboxAccountLinked method on OnDropboxAccountLinkedListener interface.

Since the MainActivity is dealing directly with DropboxAuthManager it's not necessary to return a new object
Makes LinkDropboxFragment retrieve a DropboxAuthManager onAttach
Makes DropboxAuthManager able to start the authentication process from a different context

Instead of tying the DropboxAuthManager to a context forever, now it's able to use a different context
for authentication process, the current context (Activity or Fragment) is not altered