Add delay between DTMF tones to meet carrier spec.

Carrier specifications require a specified minimum delay between DTMF
tones that are sent out. So, adding a carrier config param to add delay
between the end of processing a DTMF char and begining of processing
of the next DTMF char.
This is the same fix as ag/747687 ported for GSM connections.

BUG: 23064351
Change-Id: Icc297ed7e4bfd909dfd3fcf01b66552ff782bd17