<i18n dev> RFR: 8301875: java.util.TimeZone.getSystemTimeZoneID uses C library default file mode

[Naoto Sato]

Force opening "tzmappings" file in text mode. Confirmed the fix with customer provided test case. Also replaced the file open function with the safer one.

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Commit messages:
 - initial commit

Changes: https://git.openjdk.org/jdk/pull/23356/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23356&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8301875
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/23356.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23356/head:pull/23356

PR: https://git.openjdk.org/jdk/pull/23356