chan_pjsip: disallow PJSIP_SEND_SESSION_REFRESH pre-answer execution

This patch makes it so if the PJSIP_SEND_SESSION_REFRESH dialplan function
is called on a channel prior to answering a warning is issued and the
function returns unsuccessful.

ASTERISK-28878 #close

Change-Id: I053f767d10cf3b2b898fa9e3e7c35ff07e23c9bb