audiohal: Support stateful downmixer effects

Fraunhofer's downmixer has additional buffering, this is incompatible
with how DownmixerBufferProvider used to process incomplete buffers.

Now the effects HIDL wrapper delivers frameCount updates in
audiobuffers to the server side. This fixes playback of multichannel
sound streams.

Bug: 36181621
Test: play 5.1 track from Play Music
Change-Id: I3b2cd097d61873c6ba329f1a574235ac88b21cac