calculate appropriate audio spin loop wait using Audio profile

[atsushieno]

Currently opensles driver runs spin loop to deal with audio buffers, which is waste of CPU resources.

fluidsynth/fluidsynth/src/drivers/fluid_opensles.c

Line 257 in 5e60a75

// FIXME: this should be removed.

The usleep() adjustment is not ideal. It should take advantage of some audio hardware profiles introduced at Android 4.2 (or earlier). Research needed.

Also it may be better to introduce additional loop handler that switches to longer pooling (or sleep() instead of usleep() for long pause).

[atsushieno]

the call to usleep() is adjusted at ca29fb9 so the issue is technically gone.