ecore: make ecore_audio_out abstract

ecore: make ecore_audio_out abstract

ecore_audio does define format and source, those are then used in some
leave classes, ecore_audio is only used in the tests, and should not be
used externally. Therefore make it abstract.
The other missing implementations are in the leave classes,
They are resolved with providing empty implementations, since no format
switching is supported.

ref T5719

Reviewed-by: Cedric BAIL <cedric.bail@free.fr>
Differential Revision: https://phab.enlightenment.org/D7782