Updater: Handle ErrorCode::kUpdateAlreadyInstalled more gracefully

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