Nuplayer: Flush the decoder if seamless format change is supported

In smooth streaming enabled case, flush the decoder
if seamless format change is supported.
Otherwise,ramdomly video frames with old resolution
are queued to the new surface and results in green frames.

Calling flush ensures that all the old video frames
are cleared.

Change-Id: Ia020a01fd0eaa8af8e48b6edb526a3f53875b068