At the moment it does not look like the AGL Audiomanager configuration is entirely working.  The most visible symptom is that the module-router module to implement stream corking, etc. is not loading (not listed in "pactl list"), so overlapping playback from both the radio and mediaplayer at the same time is possible.  I had thought this might be related to agl-audio-plugin not being in the correct directory for PulseAudio 9.0, but the seemingly required change in:

https://gerrit.automotivelinux.org/gerrit/#/c/9413/

did not seem to improve things.  Audiomanager does start up, but I don't see any error messages from it or PulseAudio that point at a problem...