audio: reset vorbis EOS status after a flush

vorbis decoder reports output EOS persistently once it saw input EOS,
which causes subsequent playback last for short time if application
replays the same session by seeking to elsewhere and starting decoding
again.

Reset vorbis EOS status after a flush.

CRs-Fixed: 939213
Change-Id: I90b0cafe185e1c492d21af4e94e8a41eb3eae19a