superproject: pass groups to ToXml method.

Added the following methods to XmlManifest class.
+ GetDefaultGroupsStr() - return 'default,platform-' + platform.system().lower()
+ GetGroupsStr() - Same as gitc_utils.py's _manifest_groups func.

+ Replaced gitc_utils.py's_manifest_groups calls with GetGroupsStr.
+ Used the above methods to get groups in command.py::GetProjects
  and part of init.py.

TODO: clean up these funcs to take structured group data more instead
      of passing strings around everywhere that need parsing.

Tested the code with the following commands.

$ ./run_tests -v

Tested the sync code by using repo_dev alias and pointing to this CL
and verified prebuilts/fullsdk-linux directory has all the folders.

Tested repo init and repo sync with --use-superproject and without
--use-superproject argument.

$ repo_dev init -u sso://android.git.corp.google.com/platform/manifest -b androidx-main  --partial-clone --clone-filter=blob:limit=10M --repo-rev=main --use-superproject

$ repo_dev sync -c -j32

Bug: [google internal] b/181804931
Bug: https://crbug.com/gerrit/13707
Change-Id: Ia98585cbfa3a1449710655af55d56241794242b6
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/299422
Reviewed-by: Jonathan Nieder <jrn@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Raman Tenneti <rtenneti@google.com>

gitc_utils.py

@@ -77,22 +77,6 @@ def _set_project_revisions(projects):
       project.revisionExpr = revisionExpr
 
 
-def _manifest_groups(manifest):
-  """Returns the manifest group string that should be synced
-
-  This is the same logic used by Command.GetProjects(), which is used during
-  repo sync
-
-  Args:
-    manifest: The XmlManifest object
-  """
-  mp = manifest.manifestProject
-  groups = mp.config.GetString('manifest.groups')
-  if not groups:
-    groups = 'default,platform-' + platform.system().lower()
-  return groups
-
-
 def generate_gitc_manifest(gitc_manifest, manifest, paths=None):
   """Generate a manifest for shafsd to use for this GITC client.
 
@@ -107,7 +91,7 @@ def generate_gitc_manifest(gitc_manifest, manifest, paths=None):
   if paths is None:
     paths = list(manifest.paths.keys())
 
-  groups = [x for x in re.split(r'[,\s]+', _manifest_groups(manifest)) if x]
+  groups = [x for x in re.split(r'[,\s]+', manifest.GetGroupsStr()) if x]
 
   # Convert the paths to projects, and filter them to the matched groups.
   projects = [manifest.paths[p] for p in paths]
@@ -166,7 +150,7 @@ def save_manifest(manifest, client_dir=None):
   else:
     manifest_file = os.path.join(client_dir, '.manifest')
   with open(manifest_file, 'w') as f:
-    manifest.Save(f, groups=_manifest_groups(manifest))
+    manifest.Save(f, groups=manifest.GetGroupsStr())
   # TODO(sbasi/jorg): Come up with a solution to remove the sleep below.
   # Give the GITC filesystem time to register the manifest changes.
   time.sleep(3)