Fix regression in FastMixer statistics

At some point, the mSampleRate in FastMixerDumpState stopped being
initialized correctly.  I'm not sure when this happened, it doesn't seem to
be introduced in recent CLs.  This lack of initialization caused
some of the FastMixer statistics based on sample rate to be useless.

Change-Id: Id2a96d606130a90c4c4f1bddd59778f6c6428a9c