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>

subcmds/init.py

@@ -267,7 +267,7 @@ to update the working directory files.
 
     groups = [x for x in groups if x]
     groupstr = ','.join(groups)
-    if opt.platform == 'auto' and groupstr == 'default,platform-' + platform.system().lower():
+    if opt.platform == 'auto' and groupstr == self.manifest.GetDefaultGroupsStr():
       groupstr = None
     m.config.SetString('manifest.groups', groupstr)