urisourcebin: Set source element to READY before querying it
Generating the source element is done when urisourcebin is doing the READY to PAUSED state change, so it is reasonable to set the new source element to that state.
This also allows detecting early failures with backing libraries or hardware (checks done in NULL->READY).
Finally it makes more sense to have an element in READY when attempting to query information from it (such as SCHEDULING queries or probing live-ness).