fix visualizer inactivity detection

Current method implemented by the visualizer to detect that audioflinger has
stopped providing audio buffers does not work if the application
reads pcm captures too fast.

The fix consist in implementing a method based on real time measurement only.
One drawback is that the new method makes use of system calls that add some
overhead to the process and capture functions.

Change-Id: I53bd596b856f1cc7f0f47e08413af3335227100b