chan_unistim: Fix code, causing all incoming DTMF sent back to asterisk

Current implementation of ast_channel_tech send_digit_begin hook uses
same function for tone playback as key press handler. This cause every
incoming dtmf send back to asterisk. In case of two unistim phones
connected to each other, it'll cause indefinite DTMF loop. Fix add
separate function for dtmf tone phone play.

Change-Id: I5795db468df552f0c89c7576b6b3858b26c4eab4