Generalize LimitedWidthRequirement into a more reusable
UserPreferenceRequirement that can be used by both the limited width feature and
the persistent pinning feature (and possibly others in the future).
* Removes existing logic that checks whether the option is not null. Given that
skin.json sets the default [1], presumably this isn't needed.
* Adds unit test
[1] 65af26a258/skin.json (L163)
Bug: T322051
Change-Id: I7f228cf81a65b2eb22dbe94d2384b6c9f6da91f2
