echo-cancel: Convert AGC API to deal with pa_volume_t

echo-cancel: Convert AGC API to deal with pa_volume_t

It is expected that the underlying AGC mechanism will likely provide a
single volume for the source rather than a per-channel volume. Dealing
with per-channel volumes just adds complexity with regards to the
actual volume setting (depending on whether volume sharing is enabled or
not, we would set the volume on the source output of the virtual source,
and their sample specs may be different).

Using a single volume allows us to sidestep this problem entirely.