V4L2Decoder: streamon the output queue at flush when it's streaming

Originally we always streamon the output queue after flush is done.
However the output queue might not be streaming before flush. In this
case we don't need to streamon the output queue.

Bug: 170199771
Test: android.media.cts.AdaptivePlaybackTest

Change-Id: I94f76a5737b7f6536acc169876cf0f7e9060d5bb