The extradata is generated by encoding a dummy frame, then reset
the encoder state by mediacodec flush(). It only works for pixel
format other than AV_PIX_FMT_MEDIACODEC, since I'm not sure how
to create a dummy frame safely with AV_PIX_FMT_MEDIACODEC.
Signed-off-by: Zhao Zhili <zhilizhao@tencent.com>