Step clock animation: fix RTL issues and clean-up a little

- Leaves the bounds change animation be taken care of, by a
  `ChangeBounds` transition. This takes care of the clock moving
  correctly in RTL, and works well together with the burn-in animation
  (offset + scale) that happens when going from/to AOD.
- Changes the step clock animation logic to only update glyph offsets,
  instead of bounds + offsets.
- Removes the cancellation logic as it wasn't being used now, and was
  making the code more complicated to read.
- Renames a few methods, adds documentation, and tries to simplify the
  code a little bit.

Fixes: 274892412
Test: Manually - Going from/to AOD and posting/clearing notifications
      on lockscreen.
      Also took before/after videos and compared side by side.
Change-Id: I92f5c822a6b45ef5e4c5db680f9c795b3d8a5042