aac: correctly map 7.1ch-wide AAC from FDK AAC encoder

FDK AAC encoder outputs SCE(front)+CPE(front)+CPE(back)+CPE(back) on
MODE_7_1_REAR_SURROUND configuration.
Since decoder couldn't properly map 4 back channels, decoding failed
unless -request_channel_layout 0x8000000000000000 has been specified.
Now we treat first CPE(back) as CPE(side) on channel mapping.

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>