libfdk-aacdec: Decode the first AAC frame to reliably identify the bitstream

For implicit signaling cases (as possible for Spectral Band Replication
and Parametric Stereo Tools), the decoder must decode the first frame to
correctly identify the stream configuration (as called from
avformat_find_stream_info). The mechanism for this is built-in and only
requires adding CODEC_CAP_CHANNEL_CONF to the libfdk-aacdec AVCodec
struct.

Signed-off-by: Omer Osman <omer.osman@iis.fraunhofer.de>
Signed-off-by: Martin Storsjö <martin@martin.st>