<i18n dev> RFR: 8302983: ZoneRulesProvider.registerProvider() twice will remove provider

[Madjosz]

Fixes JDK-8302983 (and duplicate JDK-8302898)

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Commit messages:
 - 8302983: ZoneRulesProvider.registerProvider() twice will remove provider

Changes: https://git.openjdk.org/jdk/pull/12690/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12690&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8302983
  Stats: 53 lines in 2 files changed: 49 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/12690.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12690/head:pull/12690

PR: https://git.openjdk.org/jdk/pull/12690