flvmux: specify we only accept raw AAC in template caps
flvmux: specify we only accept raw AAC in template caps

No header seems to be added, and the codec ID is the same as used
for raw by flvdemux, so raw seems the only supported case.

https://bugzilla.gnome.org/show_bug.cgi?id=665394

freedesktop-unofficial-mirror/gstreamer-sdk__gst-plugins-good,matsu/gst-plugins-good,freedesktop-unofficial-mirror/gstreamer-sdk__gst-plugins-good,matsu/gst-plugins-good,freedesktop-unofficial-mirror/gstreamer-sdk__gst-plugins-good,freedesktop-unofficial-mirror/gstreamer-sdk__gst-plugins-good,matsu/gst-plugins-good,matsu/gst-plugins-good