stagefright: do not offload LD-AAC decoding

For now, do not offload LD and ELD AAC decoding because there
is no way to know if it is supported by the audio DSP implementation.
The longer term fix will be to have mapMimeToAudioFormat() use the
audio object type in track metadata to refine the AAC format and the
audio HAL list supported AAC profiles.

Change-Id: Iaa88ecf3f4ae42ad48c1b42a9b007dd80eb88147