libstagefright: Instantiate FLACDecoder at runtime

Remove the entry corresponding to FLACDecoder from
media_codecs.xml and instantiate it in OMXCodec for
mimetype "audio/flac" at runtime.

Use Google's RAW decoder to support ACodec path as
FLACDecoder is not OMX compliant.

CRs-Fixed: 749235
Change-Id: Ia8842256d00dd53293335535d3b03a3beffb13aa