mk_payload: create zero-filler as sparse file

Bug: n/a
Test: MicrodroidHostTestCases
Test: $ echo asdf >> my.apk
      $ mk_payload config payload.img (with my.apk)
      $ du payload.img.0
       0

Change-Id: If7b298e307cc8cee3a4b0db3d3db0e595db83a8c
1 file changed