tracker: ensure ID is always obtained
tracker: ensure ID is always obtained

GRL_METADATA_KEY_ID is almost a mandatory key, and thus must be obtained
even if user doesn't request it.

https://bugzilla.gnome.org/show_bug.cgi?id=758654

jasuarez/grilo-plugins,grilofw/grilo-plugins,MikePetullo/grilo-plugins,MikePetullo/grilo-plugins,MikePetullo/grilo-plugins,GNOME/grilo-plugins,grilofw/grilo-plugins,GNOME/grilo-plugins,jasuarez/grilo-plugins