frameworks/av: Fix include dir for audio defs

Currently, audio defs header file is included only when
AUDIO_FEATURE_ENABLED_EXTN_FLAC_DECODER is set to true.
By including header directory after vendor-board-platform
identification, audio defs header file is visible appropriately.

Change-Id: I16b7c3de4d095c3b945b38876f0d7a4c230a72e2
CRs-Fixed: 797456