Configure the A@H TX media player with one string

Music2 would prefer a configuration interface that takes a single
blob of data passed in from the RPC client and then passes it
opaquely to the media player.  This eliminates the need for Music2
to rev their application if we add more configuration options to
the TX player.

Change-Id: I9008dab6ab94316a6ca4c1ac605bd026545bc28e