Update written size correctly in callback thread

The original refactor did not update buffer.size correctly before
calling releaseBuffer leading to skipping issues when the requested
size was not provided. This fixes this issue.

Bug: 204055298
Test: Identical callback trace for YTMusic
Test: YTMusic plays back successfully
Test: OpenSLES pathway in Oboe Tester has expected output
Change-Id: Ifbf177ae43fa4c2a61027af6c131846961c09cd8