Add Benchmark test

[philburk]

Add a "benchmark test that would just run N voices at full speed to generate S seconds worth of audio. The wall clock time may be higher or lower than S. We could measure and report the time as a name=value pair for easy parsing.

To run a benchmark test on CPU3 you would:

adb shell synthmark -tb -n32 -s20 -c3

It would print:

benchmark.time.msec = 12345

See internal Feature Request b/37128772