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Store sparse images in the target-files, and use those (when they're
available) for building block OTAs.

- New script add_img_to_target_files is added to make the images and
  add them to the IMAGES/ subdir in the target-files.  It gets run
  from the Makefile when building a target-files.

- img_from_target_files becomes mostly vestigial: it creates the
  img.zip by just copying the images out of the target-files.  (It
  still knows how to build images for use on older target-files.)

- ota_from_target_files uses images from the target-files in
  preference to rebuilding images from the source files.

- sign_apk_target_files builds images and includes them in its output
  target files (even if the input target-files didn't have them).

Bug: 16488065
Change-Id: I444e0d722d636978209467ffc01750a585c6db75