update_attempter: Set is_install_ to false when doing a rollback.

We weren't setting is_install_ when performing a rollback, meaning it
was potentially left uninitialized or had a stale value from a previous
install/update attempt. Explicitly set is_install_ in Rollback() and in
the UpdateAttempter constructor.

BUG=chromium:905704
TEST=Add a unittest.

Change-Id: I19b646d689cab869a2aca0cd82def252c370184b
Reviewed-on: https://chromium-review.googlesource.com/1359109


Commit-Ready: Colin Howes <chowes@google.com>
Tested-by: Colin Howes <chowes@google.com>
Reviewed-by: Amin Hassani <ahassani@chromium.org>