Bluetooth: Audio: Add maintainers/codeowners

Add Thalley (emil.gydesen@nordicsemi.no) and asbjornsabo
(asbjorn.sabo@nordicsemi.no) as additional maintainers/codeowners for
bluetooth audio.

(Also adds empty audio.h file, that will later be expanded, so that
the include/bluetooth/audio directory now listed in CODEOWNERS
actually exists.)

Signed-off-by: Asbjørn Sæbø <asbjorn.sabo@nordicsemi.no>