RFR: 8332807: Parallel: Make some APIs in ParMarkBitMap private

[Albert Mingkun Yang]

Trivial moving methods from public to private.

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Commit messages:
 - pgc-api

Changes: https://git.openjdk.org/jdk/pull/19360/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19360&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8332807
  Stats: 38 lines in 1 file changed: 19 ins; 19 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/19360.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19360/head:pull/19360

PR: https://git.openjdk.org/jdk/pull/19360