Throttle MixerThread data pull to no more than twice expected rate

This helps prevent underruns with NuPlayer and other applications
which set up buffers that are close to minimum size or use deep
buffers, and rely on a double-buffering sleep strategy to fill.

Enabled by default.  Disabled by setting af.thread.throttle 0

Bug: 19062223
Bug: 21198655
Change-Id: Ia52b48e0c99588af5db53c43fede2afd775b8899