[Share 2.0] Use full share id (providerId:shareId)

Now that we support multiple managers we communicate shares to the
outside as 'providerId:shareId'. This makes sures that id's are unique
when references from the OCS API.

However, since we do not want to break the OCS API v1 we need to
somewhat hack around this.

When we switch to OCS API v2 (which we should when we support more
custom providers). We will change the id to always be the fullShareId.