<i18n dev> RFR: 8303440: The "ZonedDateTime.parse" may not accept the "UTC+XX" zone id

[Naoto Sato]

This change is to fix a regression caused by the fix to [JDK-8234347](https://bugs.openjdk.org/browse/JDK-8234347). The previous fix was simply disabling offset-based parsing if the parser was text-based. Instead, check if it is an offset or not by explicitly comparing the character with '+'/'-' and continue offset parsing if it is.

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Commit messages:
 - 8303440: The "ZonedDateTime.parse" may not accept the "UTC+XX" zone id

Changes: https://git.openjdk.org/jdk/pull/12868/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12868&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303440
  Stats: 81 lines in 2 files changed: 79 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/12868.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12868/head:pull/12868

PR: https://git.openjdk.org/jdk/pull/12868