<i18n dev> RFR: JDK-8316435: sun.util.calendar.CalendarSystem subclassing should be restricted [v5]

[Justin Lu]

> Please review this PR which restricts sub-classing of the internal calendar system in sun.util.calendar to only the existing implementations. Drive by cleanup included.
> 
> As the implementation is long-standing and complete with no intent for future sub-classing, the CalendarSystem should be made sealed. Modifiers adjusted accordingly (`JulianCalendar.Date` must now have package visibility).
> 
> 
> This system has the following structure,
> 
> `CalendarSystem` extended by `AbstractCalendar` extended by `BaseCalendar` extended by 
> (`Gregorian, JulianCalendar, LocalGregorianCalendar`)
> 
> `CalendarDate` extended by `BaseCalendar.Date` extended by 
> (`Gregorian.Date, ImmutableGregorianDate, JulianCalendar.Date, LocalGregorianCalendar.Date`)
> 
> Additionally, CalendarUtils was made `final`, as it is a utility class composed of static util methods.

Justin Lu has updated the pull request incrementally with two additional commits since the last revision:

 - cleanup CalendarDate after revert
 - Revert "Replace sprintf0d with Formatter"
   
   This reverts commit 84a346aed2be262b717f82fbbc32a4ed0323bccc.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/15803/files
  - new: https://git.openjdk.org/jdk/pull/15803/files/790f7506..aaee9aa9

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15803&range=04
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15803&range=03-04

  Stats: 85 lines in 7 files changed: 45 ins; 5 del; 35 mod
  Patch: https://git.openjdk.org/jdk/pull/15803.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15803/head:pull/15803

PR: https://git.openjdk.org/jdk/pull/15803