sample_images: Support sparse files.

The sample images are mostly sparse space. This patch stores those
files as sparse files in the .tar.bz2 files and simplifies the rules
to extract them directly onto the destination directory.

acp still doesn't support sparse files, so when these files get copied
to the /data partition, they won't be sparse files. Nevertheless, this
patch is still required.

Bug: 26955860
TEST=mmma showcommands system/update_engine; `du -hs` the intermediate files.

Change-Id: I24a2c1f7a28ee79cc545650e04705a22d7fc7cdb
diff --git a/sample_images/generate_images.sh b/sample_images/generate_images.sh
index 1c2a83c..17bb11c 100755
--- a/sample_images/generate_images.sh
+++ b/sample_images/generate_images.sh
@@ -184,7 +184,7 @@
   # Generate the tarball and delete temporary images.
   echo "Packing tar file sample_images.tar.bz2"
   tar -jcf "${OUTPUT_DIR}/sample_images.tar.bz2" -C "${OUTPUT_DIR}" \
-    "${IMAGES[@]}"
+    --sparse "${IMAGES[@]}"
   cd "${OUTPUT_DIR}"
   rm "${IMAGES[@]}"
 }