aaudio: use actual DSP burst size in timing model

This will more accurately track the read or write
position of the DSP.

The client sometimes aggregates multiple hardware bursts
to avoid waking up too often. This can make it look like the DSP has
processed a larger amount of memory than it really has.
With this change, the timing model will advance in sync with the hardware.

Bug: 117834966
Bug: 130911274
Test: see bug for repro steps
Change-Id: Id22f1f313e02f0514665ee5ac144cab4684e3bc1