Fix flac encoder and decoder EOS handling

When the flac encoder saw EOS on its input, it would output an empty
buffer with EOS set, without decoding the current buffer or flushing
pending output buffers. The flac encoder also didn't output CSD, making
its output unsuitable for feeding back into the flac decoder.
When the flac decoder saw EOS, it would tag the next output buffer
with EOS, but didn't flush pending output buffers.

Bug: 75963284
Test: CTS
Change-Id: I53ac2f26fe77e50c899587fc62fc66cf0b85d167