OpenSL ES: Don't store or return the adjusted volume level.

According to our own docs we should always return 1.0 (MAX) volume level
if the device doesn't support setting the volume, which is the case
for OpenSL ES on Android.

Task-number: QTBUG-42159
Change-Id: I9144e9e2cd84cc80633c68c5d6fa32b8dbd0fcd5
Reviewed-by: Yoann Lopes <yoann.lopes@theqtcompany.com>
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>
Check whether current context can be obtained when deleting textures.

Add a warning in that case.

Task-number: QTBUG-41966
Change-Id: I491ee9e13c9504040ab789951656983a736d9203
Reviewed-by: Yoann Lopes <yoann.lopes@theqtcompany.com>