Fixed underrun in audioflinger mixer.

Fixed underrun in audioflinger mixer.

When all audio tracks have been disabled and the mixer is running idle before the output stream is placed in standby,
the mixer sometimes fails to write to the output stream on time to avoid underrun.

This is because the sleep period used to wait before the next write to output stream is too close to the actual buffer duration.
In fact this sleep time is not critical as if we write too early to the output stream, the kernel driver will wait for free buffers
from the audio DSP DMA and we will sleep anyways.

The fix consists in dividing the calculated wait period by 2 to increase the margin.

Change-Id: I5730887dc2ccce2a511bc858494a6f7da6b392a0