Bug 921115 - [Clock] Lap string is not localized

- stopwatch_panel.js
    - Use navigator.mozL10n.localize to localize the 'Lap [n]' label
- stopwatch_panel_test.js
    - I think a stopwatch_panel_test.js is already in another PR (https://github.com/mozilla-b2g/gaia/pull/12454), so this one is submitted here purely for review