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>

subcmds/sync.py

@@ -694,10 +694,10 @@ later is required to fix a server side protocol bug.
       load_local_manifests: Whether to load local manifests.
     """
     if manifest_name:
-      # Override calls _Unload already
+      # Override calls Unload already
       self.manifest.Override(manifest_name, load_local_manifests=load_local_manifests)
     else:
-      self.manifest._Unload()
+      self.manifest.Unload()
 
   def UpdateProjectList(self, opt):
     new_project_paths = []