repo: fix up license text

Switch the copyright holder to "The Android Open Source Project" to
match all the other source files in the tree, and move it to the top
of the file to match everything else we do.

Change-Id: Ie15d8e2bc004a626e45f715271deeaf3919dc44a
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/256235
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: David Pursehouse <dpursehouse@collab.net>