Addresses bug 7140347.

When flushing the decoder, you need to make sure to zero out any
lingering spectral coefficients as well as zero out the overlap buffer
state after decoding the final access unit.  Zeroing out the spec
coefficients *before* you decode the last block, however, seems like a
mistake.  If the final block contains any useful information for the
stream, then you end up outputting the overlap portion of the second
to last decoded block combined with zeros.  Because of the shape of
the second to last block's windows, this results in what appears to be
a fade out, but actually is a zero'ed out final block.
Signed-off-by: John Grossman <johngro@google.com>
Change-Id: I0fab9ee1a5899811d5519a91dc05631e9bf4963c