AAudio: fix prepareBuffersForStop()

Use mAudioEndpoint->getDataWriteCounter() instead
of getFramesWritten() because the ClockModel uses
the server frame positions.

Bug: 319071878
Test: Run OboeTester and play an MMAP stream.
Test: Start and stop the stream several times and listen for
Test: excessive noise when the stream is stopped.
Test: The crash was only happening on an OEM device
Test: when this bug combined with timestamp errors on the device.
Test: See bug for more details.
Flag: EXEMPT bugfix
Change-Id: I80fd9aa428c96a90100bab3205b2b51f5475c904