stagefright: Fix metadata/message conversion issues

 * Remove duplication of code between ExtendedCodec and FFMPEGSoftCodec,
   just call into ExtendedCodec and properly ifdef QCOM-only parts.
 * Fix CSD not being converted when AV_ENHANCEMENTS wasn't set- this
   was breaking the software video codecs on Hammerhead.

Change-Id: I9cd4316ce19b15baabf12b78b992498ce48e2697