opensles: implement capacity using buffer queue length

[philburk]

The OpenSL ES buffer queue length controls the allocated size of the internal buffer. So we can use that to make sure there is enough capacity to protect against glitches when setting a callback size or a high capacity.

Fixes #744 It might help with #952

[robertwu1]

Feel free to submit once you addressed my remaining comments.

[robertwu1]

LGTM!