High cpu usage in AudioEncoder thread from webrtc platform

[Gary.Wang]

When no audio output is performed, profile anbox session process, we discovered that the Audio Encoder thread from the webrtc stack consumed abnormally CPU times.

```
  43.10% AudioEncoder webrtc_common.so
  11.30% network [kernel]
   4.50% EncoderQueue libcuda.so.550.40.07
   4.26% AudioEncoder [kernel]
   4.08% worker webrtc_common.so
   3.67% EncoderQueue [kernel]
   3.53% worker [kernel]
   2.92% EncoderQueue webrtc_common.so
   2.83% EncoderQueue libnvcuvid.so.550.40.07
   2.74% network webrtc_common.so
```

```
    43.10% 592 AudioEncoder webrtc_common.so [.] silk_noise_shape_quantizer_del_dec_sse4_1
     8.47% 127 AudioEncoder webrtc_common.so [.] silk_warped_autocorrelation_FLP
     6.81% 99 AudioEncoder webrtc_common.so [.] silk_resampler_down2_hp
     4.35% 61 AudioEncoder webrtc_common.so [.] silk_NLSF_del_dec_quant
     2.87% 39 AudioEncoder webrtc_common.so [.] silk_inner_product_FLP
     2.51% 25 AudioEncoder webrtc_common.so [.] silk_resampler_private_down_FIR
     2.10% 33 AudioEncoder webrtc_common.so [.] transient_analysis
     1.91% 23 AudioEncoder webrtc_common.so [.] opus_fft_impl
```

This issue is related to the silence audio effect produced by the Anbox session, which triggers the WebRTC noise shaping quantization (NSQ) module to do some heavely work