opensl: fix incorrect injected buffer

Sometimes, buffer with incorrect capacity were injected into
the audio, causing the daemon to crash.
Before the stream's split, stopStream was flushing the buffers.
In this patch, stopStream will correctly re-init the buffers and
avoid to inject incorrect buffer into the audio.
Also, remove unused value in buf_manager.h and clean some warnings
for log.
Finally, modify engineServiceRing to avoid many push operations
and make it similar to engineServicePlay

Change-Id: I589cea814452d3dbf5172af6369e448dcd9d5c72
GitLab: #676