Merge changes from topic 'nyc-mr1-ue_sideload_recovery' into nyc-mr1-dev

* changes:
  Setup a temporary directory for update_engine_sideload.
  DO NOT MERGE: Make update_engine compile in the branch.
  New setting to mark postinstall as optional.
  Report the progress of the update when sideloading.
  Compile update_engine_sideload as a static recovery program.
  Remove libcurl support from update_engine_sideload.
  Build update_engine_sideload.
  Implement a memory-based Prefs class.
  Remove unused libbrillo-http dependency.