echo-cancel: Start capture at a sane volume if we're doing webrtc AGC

This is required to make sure the capture output has sufficient energy
for the AGC to do its job.

src/modules/echo-cancel/echo-cancel.h

@@ -68,6 +68,7 @@ struct pa_echo_canceller_params {
             pa_sample_spec sample_spec;
             void *trace_callback;
             bool agc;
+            bool first;
         } webrtc;
 #endif
         /* each canceller-specific structure goes here */