Go faster by wasting CPU

Part of this test generates frames faster than SurfaceFlinger can
display them, to test SF's ability to drop stale timestamped
buffers.  On certain devices, the power management reduces the
clock frequencies to the point where we can't actually generate
frames that quickly.

This adds a spinning thread on multi-core devices to increase the
apparent workload.

Bug 10955012

Change-Id: Ic07c93765a41e150449fdce61f4fc4f3785451d0