stagefright: featurise QTI FLAC decoder-specific code

This is QTI FLAC specific code, it should be placed under
its feature tag. Change basically has the effect of
retaining the old behavior if QTI_FLAC_DECODER is not
defined.

CRs-Fixed: 804151
Change-Id: I7a7abc6736e2b28911f0922eef3c19736122860b