hal: effects: volume_listner: Accumulate effect buffer

mEffectBuffer in volume listener is overwritten instead of
accumulation. This causes only single stream to be audible
as data from other tracks copied into mEffectBuffer is lost.
Avoid memcpy and accumulate data if access mode is set to
accumulate.

Change-Id: I5658cfdac968e82180b30f18ed540c3225bd07b8