If applyPayload() fails with kUpdateAlreadyInstalled, mark update as
already installed. While this is not ideal since this might not be the
update we actually installed, it's still better than outright dying.
Test: install update, unset needs_reboot_id, kill updater, install
update again.
Change-Id: Iac264896cffd1db522d81fc2050eb71d62ca91bf