Add full-backup stream API to BackupTransport

Also started migrating the definition of transport success/failure constants
into BackupTransport to make them permanent.

The new methods are not yet plumbed in; this is just to allow forward
progress against a proposed stable API.

Bug 15329632

Change-Id: I27472e09b831350c140b9fa548ebda3af334eb1a
3 files changed