Fix SoundPool buffer size rounding error. Bug 2327620.

AudioTrack was modified earlier to calculate minimum buffer size
based on the hardware reported latency. Previously, it was a
hard-coded value. As a result of this change, the minimum buffer
size is now variable based on hardware latency. On Passion, this
brought out a subtle rounding error in the buffer size calculation
in SoundPool. This can cause AudioTrack creation to fail based on
the requested sample rate. This fix calculates the total buffer
size first, and then does rounding before dividing by the number
of buffers.