extract_utils: prefix_match(): do not strip target_args from its output

* The write_product_copy_files() and write_product_packages() functions
  rely on its undocumented behavior of keeping target_args in the
  returned list, because they are users of target_args (such as
  ";PRESIGNED" etc).
* Make the behavior documented.

Change-Id: If71595dca32abd40039706d4fed2d7f12e005365
Signed-off-by: Vladimir Oltean <olteanv@gmail.com>