audio_processing/aec: Fixes an incorrect sampling rate multiplier when processing in 48 kHz
audio_processing/aec: Fixes an incorrect sampling rate multiplier when processing in 48 kHz

In AEC a fixed fft size is used, but processing can in the lower band be in either 8 or 16 kHz.
Therefore we need a multiplier/rate factor to, for example, map frequency bands in Hz to frequency bins.

The multiplier/rate factor can only be either 1 or 2, but when 48 kHz support was added it was assigned 3.

BUG=crbug.com/482424
R=aluebs@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/43329004

Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#9117}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 3cfa756f371ffdcc0369d745e9832cfff55861ba

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