RFR: 8302983: ZoneRulesProvider.registerProvider() twice will remove provider [v2]

[Madjosz]

> Fixes JDK-8302983 (and duplicate JDK-8302898)

Madjosz has updated the pull request incrementally with one additional commit since the last revision:

  apply review comments

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Changes:
  - all: https://git.openjdk.org/jdk/pull/12690/files
  - new: https://git.openjdk.org/jdk/pull/12690/files/e87cfef8..0c7e5889

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12690&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12690&range=00-01

  Stats: 71 lines in 1 file changed: 31 ins; 33 del; 7 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