TT#14008 eliminate duplicate DTMF reports when forking media
With multiple media subscriptions, codec handlers are called
consecutively, once for each forwarding chain, leading to DTMF events
reported multiple times. The DTMF trigger must therefore keep track of
the state in the upper media object, not in the codec handlers.
Change-Id: I9ceaf406e093f25b7c037a325a0f2a7a91954922