An issue was found where a particular manufacturer's phones add a
trailing space to the end of the rtpmap attribute when specifying
a payload type that has a "param" after the format name and clock
rate. For example:

a=rtpmap:120 opus/48000/2 \r\n

Because pjmedia_sdp_attr_get_rtpmap currently takes everything after
the second '/' up to the line end as the param, the space is
included in future comparisons, which then fail if the param being
compared to doesn't also have the space.

We now use pj_scan_get() to parse the param part of rtpmap so
trailing whitespace is automatically stripped.

ASTERISK-29654

Change-Id: Ibd0a4e243a69cde7ba9312275b13ab62ab86bc1b