base: audioencoder: Keep serialize event behind buffers

If a serialized event arrives behind a buffer, it should not be send before it. This fixes the pending event handling so that only early pending events, the one that arrrived or was generated while the adapter was empty get send before pushing buffer. All other events are not pushed after.

This issue lead the latency tracer to think our audio encoder did not have any latency. This was testing with opusenc in a live pipeline.