RFR: 8289078: Make STARTTIME_ANY and STARTTIME_PROCESS_UNKNOWN fields static in ProcessHandleImpl

[Andrey Turbanov]

Fields `STARTTIME_ANY` and `STARTTIME_PROCESS_UNKNOWN` are actually unused, but they are useful as documentation on the interface to native.
Make them `static` to reduce memory usage.

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Commit messages:
 - [PATCH] Make STARTTIME_ANY and STARTTIME_PROCESS_UNKNOWN fields static in ProcessHandleImpl

Changes: https://git.openjdk.org/jdk/pull/9196/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9196&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8289078
  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/9196.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9196/head:pull/9196

PR: https://git.openjdk.org/jdk/pull/9196