[system_updater] switch system_updater to use pkg resolver

This switches the system updater to use package resolver
for caching packages, instead of talking directly to amber,
which will eventually make it easier to remove amber without
disruption.

Test: Manually ran system_ota_tests and confirmed update worked
Bug: PKG-607 #done
Change-Id: I5111f8011b5cf49e913208096529a9c7cc98dd7e