Add delay between DTMF tones to meet carrier spec.

Carrier specifications require a minimum delay of 215ms between DTMF
tones that are sent out. So, adding a delay between the end of
processing a DTMF char and begining of processing of the next DTMF char.

BUG: 23064351
Change-Id: Ife7b9dfe99319cd3fd383034d8076dd475ad5fa0