Tools: Work around problem with mail for nested service groups

At the moment, mail for service groups that have other service groups
as members is stuck in the queue as the final recipients cannot be
determined.

This change filters out nested service groups, so mail to the
containing service group is only delivered to its "ordinary" members.

Bug: 63163
Change-Id: I8546cb6d8d91073fcfe96d9351fc28493781dd8b
Merge "Tools: Work around problem with mail for nested service groups" into production