DCHECKing for deprecated 8kHz support in AGC and changing fuzzer

This CL adds a DCHECK for the deprecated 8 kHz rate in APM.
It also updates the agc fuzzer code to properly do band-split on
the signals, and not send 8 kHz signals into the AGC.

Bug: chromium:1028092,chromium:1028172
Change-Id: I1e7c8d721834310e94b0e21efea07f75da837cab
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/160600
Reviewed-by: Sam Zackrisson <saza@webrtc.org>
Commit-Queue: Per Åhgren <peah@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#29914}

modules/audio_processing/gain_control_impl.cc

@@ -375,6 +375,9 @@ int GainControlImpl::enable_limiter(bool enable) {
 void GainControlImpl::Initialize(size_t num_proc_channels, int sample_rate_hz) {
   data_dumper_->InitiateNewSetOfRecordings();
 
+  RTC_DCHECK(sample_rate_hz == 16000 || sample_rate_hz == 32000 ||
+             sample_rate_hz == 48000);
+
   num_proc_channels_ = num_proc_channels;
   sample_rate_hz_ = sample_rate_hz;