Add description and configuration methods to the transport interface

It's now possible for the transport to supply a descriptive string to inform
the user about the nature of the backup destination (e.g. telling the user
what account the data is being stored under), and to supply an Intent with
which the system can initiate a configuration Activity supplied by the
transport (e.g. allowing the user to select which account their data is
being stored under).

The transport interface is not public.

Part of bug 2753632 and bug 2987804

Change-Id: I911f70a3ea440daf2a7a04710e9d7e65b61a58db