Fix decoder EOS handling

Conceptually it should be the same whether EOS is signalled on the last
buffer holding data, or an empty buffer that follows. Make it so that
this actually behaves the same for mp3, AAC and Vorbis.

b/8747869

Change-Id: Idece8ef45689a3ffaf70fb45d19862d7b93b2f92
6 files changed