update_engine: Add XZ compression for CrOS

This patch implements XZ compression for Chrome OS. It is only activated
for only delta payload at this point. The XZ compression is typically
slower than Bzip2 to compress, but faster to decompress. However, it
uses slightly higher memory in decompression. It compresses about 5%-10%
better than the current Bzip2.

BUG=chromium:758792
TEST=unittests pass; brillo_update_paylod {generate|verify} passes;

Change-Id: Icdaf1d37d344c30a796845eb64daa00120f3598a
Reviewed-on: https://chromium-review.googlesource.com/811895


Commit-Ready: Amin Hassani <ahassani@chromium.org>
Tested-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Alex Deymo <deymo@chromium.org>
Reviewed-by: Ben Chan <benchan@chromium.org>