Allow executing update_engine_sideload from recovery.

The recovery flow for A/B devices allows to sideload an OTA downloaded
to a desktop and apply from recovery. This patch allows the "recovery"
context to perform all the operations required to apply an update as
update_engine would do in the background. These rules are now extracted
into a new attributte called update_engine_common shared between
recovery and update_engine.

Bug: 27178350

(cherry picked from commit d63084d3)

Change-Id: I1f3e1e83a21e37e09b69cd9c497f87b42b9cbeb1