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
Merge "Fix decoder EOS handling" into du44