rtspsrc: make all srtp encoder properties explicit

rtspsrc: make all srtp encoder properties explicit

The Session Data Protocol doesn't allow specifying a cipher for the
SRTCP, so it will use the SRTP one. In the "srtpenc" element the cipher
"aes-128-icm" is the default for SRTP and SRTCP, but if we want to have
an SRTCP with the "aes-256-icm" cipher then we also need to set the SRTP
cipher to "aes-256-icm", otherwise "aes-128-icm" will be used instead.

https://bugzilla.gnome.org/show_bug.cgi?id=767799