The public "IGroupManager" service returned by the dependency injection
system is automatically initialized with an "OC\Group\Database" backend.
However, no backend is automatically set in private "GroupManager"
instances. Therefore, a private "GroupManager" instance does not work as
expected when initialized through the dependency injection system.
Due to that this commit reverts a previous change in which the public
"IGroupManager" was replaced by a private "GroupManager" instance. That
change was needed when strict types were set, as "getSubAdmin()" is not
part of "IGroupManager" API, so the type had to be changed to
"GroupManager". Until a better solution is found strict types are
disabled again to be able to inject "IGroupManager" and also use
"getSubAdmin()".
Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>