uridecodebin: Don't force floating reference for future reusable decodebin

uridecodebin assumes that refcount of decodebins stored in pending_decodebins
are floating but it might not be true in case that refcount of the decodebin
was touched in other places. To avoid the floating refcount issue,
hold strong reference.

Part-of: <gstreamer/gst-plugins-base!1119>
33 jobs for !1119 with backport-mr-1113-into-1.18-uridbin-refcount-fix in 29 minutes and 14 seconds (queued for 18 seconds)
merge request