AGC: Change default clipping level min to 70

The old value was 170, but experiments have shown that 70 is better.

This will let the AGC reduce the gain further when input clipping is
detected. The effect should be less clipping, but sometimes slightly
lower signals.

In Chrome, the value 70 has already been used since June (see
https://codereview.chromium.org/2928133002).

Bug: webrtc:6622, chromium:672476
Change-Id: Ie5a60bb875eef71f303b28e096b22a8cd4b449d4
Reviewed-on: https://webrtc-review.googlesource.com/20222
Reviewed-by: Per Åhgren <peah@webrtc.org>
Commit-Queue: Henrik Lundin <henrik.lundin@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20563}

modules/audio_processing/include/audio_processing.h

@@ -110,7 +110,7 @@ static const int kAgcStartupMinVolume = 85;
 #else
 static const int kAgcStartupMinVolume = 0;
 #endif  // defined(WEBRTC_CHROMIUM_BUILD)
-static constexpr int kClippedLevelMin = 170;
+static constexpr int kClippedLevelMin = 70;
 struct ExperimentalAgc {
   ExperimentalAgc() = default;
   explicit ExperimentalAgc(bool enabled) : enabled(enabled) {}