Report the progress of the update when sideloading.

update_engine_sideload now accepts a file descriptor number via a new
--status_fd flag. When applying the update, it will report back to
recovery via this file descriptor the progress of the update in the
format expected by recovery.

This patch also initializes the Subprocess singleton used during the
update.

Bug: 27178350
TEST=Executed update_engine_sideload from recovery and watched message updates.

Change-Id: I40f5452c9220530651e8cd6cde681e39dd0a6aad