RFR: 8300916: Re-examine the initialization of JNU Charset in StaticProperty

[Naoto Sato]

This issue was found during the review of this PR: https://github.com/openjdk/jdk/pull/12132 where `Charset` class was loaded/initialized at the phase 1 of the startup process. Since `Charset` depends on `StaticProperty`, loading of `Charset` class should be delayed. I basically moved cache for `jnuCharset` into the actual calling locations `ProcessImpl` and `ProcessEnvironment` for unix platforms so that initPhase1() won't initialize `Charset` class.
Unrelated, but I replaced `Locale.ENGLISH` with `Locale.ROOT` in the argument of `toLowerCase()`.

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Commit messages:
 - 8300916: Re-examine the initialization of JNU Charset in StaticProperty

Changes: https://git.openjdk.org/jdk/pull/12171/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12171&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8300916
  Stats: 27 lines in 3 files changed: 4 ins; 12 del; 11 mod
  Patch: https://git.openjdk.org/jdk/pull/12171.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12171/head:pull/12171

PR: https://git.openjdk.org/jdk/pull/12171