src & test: Destroy MediaBuffer after it's been sent

Newer versions of Android expose a refcounting problem where MediaBufferBase objects tied to an observer don't get released properly, possibly because of signalBufferReturned not getting called.

As there is always one last remaining reference on the buffer and we're ultimately done processing it, just steal & destroy it.

Requires this change as a prerequisite, as implicit release() will never release the resource in question with broken refcounting: https://github.com/Halium/libhybris/pull/13