Inject public IGroupManager instead of private GroupManager

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>