aaudio: use minimum of one burst in buffer

The old minimum was zero, which minimized latency
but was probably too aggressive for some devices.
It is better to avoid glitches.
Because of other changes in AAudio MMAP, this will
have the same latency as earlier version,
but better immunity from glitches.

Bug: 153589709
Test: launch OboeTester
Test: tap TEST OUTPUT
Test: tap OPEN and START
Test: notice that actual buffer size will be 1 * burstSize
Change-Id: Ie53352a0589c710be11f74e995bd12be4f7c45a8