OpenSL ES: improve buffer logic

Don't use relaxed load and stores, we need to be stricter to avoid
problems with high frequency re-fills of the buffer. If we don't enforce
ordering we might end-up spending more time trying to acquire an open
slot in the buffer. Updating processes bytes is also moved off the
"OpenSL" thread.

Added some comments for improved readability.

Change-Id: Ie27965fc6bf4b8394081ae6419f4933522ada98e
Reviewed-by: Yoann Lopes <yoann.lopes@theqtcompany.com>