Fix Appstream validation

The XML spec requires the XML declaration to be the first line of the
file.  This is causing an error in "appstreamcli validate" and also
xmllint.  This CL moves the copyright header comment to the line after
the XML declaration (similar to how the copyright header comes after the
shebang in shell scripts).

R=thestig

Fixed: 336911496
Change-Id: I6196c985fbd8099625b0ca3a1107c04b0ba87782
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5617599


Auto-Submit: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1313096}