Simplify timezone-related code for user preferences

This patch simplifies and fixes a few issues in code related to the
'timecorrection' user setting:
- Always re-apply UserTimeCorrection normalization to the preference
  value, although it should be already normalized
- Avoid duplicating code from UserTimeCorrection, both for the
  pipe-splitting and the offset computation/fallback
- Use better variable names
- Inject an ITextFormatter for generating the dropdown options, instead
  of a ContextSource (ew) or a Language, removing calls to wfMessage as
  well. Note that the ITextFormatter is not injected into the
  preferences factory because the eventual goal is to move the code to a
  new HTMLFormField class.
- In TimezoneFilter, remove a redundant check: the value comes from the
  form, and the option for using the system time is always "System|XXX",
  never just "System". This seems to have been introduced in
  I2cadac00e46dff2bc7d81ac2f294ea2ae4e72f47; the previous code was only
  comparing $data[0], and not $tz. Change the test accordingly and add a
  test case.
- Add missing star to docblocks in UserTimeCorrection, as well as a
  missing int cast.
- Fix typo and other style issues in UserTimeCorrectionTest
- Bonus: add missing docblock star in HTMLApiField

Bug: T309629
Change-Id: Iab35eb17259826429e4b6bc1ba7385ab57884e98

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