LfsBackend: Make constructor and DEFAULT private

Make the constructor private and abstract the creation of instances
to static methods so that callers don't need to explicitly pass null
to create the default instance.

Make DEFAULT private and introduce a name() method that returns the
name, or DEFAULT if name is null.

The hashCode method unnecessarily converts null to the DEFAULT value,
so change that to just use the value (potentially null) as is.

Change-Id: Ibac02e42217e498f6101700660b085a23005f2b3
4 files changed