res/res_rtp_asterisk: fix skip in rtp sequence numbers after dtmf

When generating dtmfs, asterisk can incorrectly think packet loss
occured during the dtmf generation, resulting in a jump in sequence
numbers when forwarding voice frames resumes.  This patch forces
asterisk to re-learn the expected sequence number after each DTMF
to avoid this

ASTERISK-29869 #close

Change-Id: Icc7de3d947b207b82c99d3c327af8095884df853