update_engine: Reset update pending callback after update actions

Reset the update pending callback after update actions have been
carried out, this is so that the current forced update status are
available to the policies that are called in UpdateActions.

BUG=chromium:852860
TEST=Tested updating locally

Change-Id: I44a70b408c83c2adea4cd8c386f777c4799a19f5
Reviewed-on: https://chromium-review.googlesource.com/1164527


Commit-Ready: Adolfo Higueros <adokar@google.com>
Tested-by: Adolfo Higueros <adokar@google.com>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Sen Jiang <senj@chromium.org>