fdkaacdec: Use WAV channel mapping instead of interleave setting

The latter is going away in libfdk-aac 2.0.0. Instead, MPEG-style output
is always non-interleaved and WAV-style output is always interleaved.
Earlier libfdk-aac also defaults interleaving accordingly.

Since our reordering looks at the associated PCE indices instead of the
actual channel order, we're agnostic to the mapping.

For gstreamer/gst-plugins-bad#825
4 jobs for fdkaac-2 in 6 minutes and 52 seconds
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