compositemixer: Set idle callback priority to default
compositemixer: Set idle callback priority to default

Change-Id: I4f66e564eebd8fc35435e370662bf6ec37680bf2

Kurento/kms-core,shelsonjava/kms-core,KurentoLegacy/gst-kurento-plugins,TribeMedia/kms-core,KurentoLegacy/gst-kurento-plugins,Kurento/kms-core,TribeMedia/kms-core,Kurento/kms-core,ESTOS/kms-core,Kurento/kms-core,TribeMedia/kms-core,KurentoLegacy/gst-kurento-plugins,ESTOS/kms-core,ESTOS/kms-core,ESTOS/kms-core,shelsonjava/kms-core,TribeMedia/kms-core,shelsonjava/kms-core,shelsonjava/kms-core,KurentoLegacy/gst-kurento-plugins