Add a couple of transport-descriptive methods to IBackupManager

Privileged callers can now ask the transport for a string describing
its current state, and for an Intent that can be passed to startActivity()
in order to bring up its exported configuration UI.  These will be used
in Settings in order to e.g. show the user the currently active account
being used for backup, and allow the user to choose an account.

The data being funnelled through IBackupManager here are the ones
already exposed by the transports in their implementation of the
IBackupTransport interface.

Bug: 2753632

Change-Id: I2227a2b111d8d0ddf221d63020e20c1316fff55b