Issue: #9183
RFC2326 Section C.1.1 specifies that the URI to identify a track can be either
absolute (like rtsp://example.com/path) or relative (like "path"). Currently
we don't handle absolute URI, and this CL is to add the support.
Note though, we don't currently use the Content-Base or Content-Location
headers for the session URI.
PiperOrigin-RevId: 384649818
