libfdk-aacdec: Support building with the latest version of fdk-aac

The latest fdk-aac code drop (from android 5.0) changed the channel
layout enums (changing the value of existing enum constants), and
renamed the option for downmixing.

The failsafe comparison between ctype and FF_ARRAY_ELEMS(channel_counts)
can trigger warnings (-Wtautological-constant-out-of-range-compare)
when building with the old FDK AAC releases, where it can't be
out of range with the enum values used there.

CC: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>