aaudio: set default buffer size to capacity/2

A recent change in master caused the default buffer size to be zero. ag/9499599
That caused a CTS failure.
This change restores the behavior of Q.

Bug: 143308870
Test: atest CtsNativeMediaAAudioTestCases
Change-Id: Id70b0c53e80531c3ea491b56b8fc107ca5ab6e51
1 file changed