Write header file with correct extradata

When recording, the header must be written with extradata set to the
content of the very first packet.

Suggested-by: Steve Lhomme <robux4@ycbcr.xyz>

Fixes <https://github.com/Genymobile/scrcpy/issues/351>
Fixes <https://github.com/Genymobile/scrcpy/issues/416>