Corrected crypto tag in SDP ANSWER for SRTP. (again)

The original fix (r380043) for getting Asterisk to respond with the correct
tag overlooked some corner cases, and the fact that the same code is in 1.8.

This patch moves the building of the crypto line out of
sdp_crypto_process(). Instead, it merely copies the accepted tag. The call to
sdp_crypto_offer() will build the crypto line in all cases now, using a tag of
"1" in the case of sending offers.

(closes issue ASTERISK-20849)
Reported by: José Luis Millán
Review: https://reviewboard.asterisk.org/r/2295/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@380347 f38db490-d61c-443f-a65b-d21fe96a405b