Clamp decoder sample rate to 32000 in iSAC

We want to crate the illusion that iSAC supports 48000 Hz decoding,
while in fact it outputs 32000 Hz. This is the iSAC fullband mode.

Currently this is (also) handled by higher layers, but in future
changes this will not be the case.

R=tina.legrand@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/47809004

Cr-Commit-Position: refs/heads/master@{#8889}
1 file changed