manifest_xml: Add Load and Unload methods

- do not call the internal method from subcmds/sync.py.
- use the correct default groups for submanifests.
- only sync the superproject when we are told to.

Change-Id: I81e4025058f1ee564732b9e17aecc522f6b5f626
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/334639
Reviewed-by: Mike Frysinger <vapier@google.com>
Reviewed-by: Raman Tenneti <rtenneti@google.com>
Tested-by: LaMont Jones <lamontjones@google.com>