Bug 902544 Refactor rendering methods for Analog clock

1. Simplifies computation of angles for `setTransform`. Fractional seconds, minutes, hours are passed to `setTransform` which computes the resultant angle.
3. Removes checks against repainting clocks, which didn't seem to be live code. May close [Bug 817993](https://bugzilla.mozilla.org/show_bug.cgi?id=817993).
4. Caches element selection and moves animated element creation out of the update code.