aaudio: do not force legacy burst to callback size

That was causing glitching when PerformanceMode==NONE
because the bufferSize was two bursts, which could be much
smaller than the actual mixer period.

BUG: 165675305
Test: see bug for detailed repro steps
Test: atest CtsNativeMediaAAudioTestCases
Change-Id: I5d9b8d07d8b4dbb81ea2f126444aac6472d3107b