It turns out I misunderstood which media_player services are available
when a media player supports StepVolume. This PR maps the Alexa
StepSpeaker messages to the volume_up and volume_down services.
Currently Alexa allows you to specify the number of steps but the media
player volume_up and volume_down services don't support this. For now I
just look to see if the steps are +/- and call up/down accordingly.