audio: Enable FFV for APQ8909 target

Define feature flags to enable FFV.
Update sound_trigger_platform xml file structure to match with sthal.
Take latest mixer paths and platform xml files in AHAL and STHAL.
Set flag to not to compile legacy STHAL for 8909.

CRs-Fixed: 2273661
Change-Id: Iaf43d9ad0645c4aea7a46401a084cef4a94b2d13