previously, dtmf_inject wouldn't add the events as the dtmf_payload_type
was set to -1 as part of the delay buffer handling. However, as we
know we want this DTMF, this change updates the function to check for
and use the real_dtmf_payload_type value
Then, in codec_add_dtmf_event, we can check the injected bool and
if set, use the real_dtmf_payload_type again to decide whether to
push the event onto the queue
closes#1722
Change-Id: I34e35f92e014f4aed9e045737df7400155b86602