liblp: Expose kDefaultGroup.

kDefaultGroup is the 'default' group that is never deleted
and has no size constraints. During a virtual a/b update,
the following series of action happens:
- move partitions out of groups that will be shrunk / deleted
  (into default)
- shrink / delete these groups
- grow / add other groups
- move those partitions to their destination group.

Otherwise, the metadata is inconsistent.

Bug: 138816109
Test: builds

Change-Id: I57cf4b44dda948377c6b3153756f469caa4652bc