RFR: 8330339: G1: Refactor G1BlockOffsetTable APIs

[Guoxiong Li]

Hi all,

This patch moves several methods to the `private` part.

Thanks for taking the time to review.

Best Regards,
-- Guoxiong

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Commit messages:
 - JDK-8330339

Changes: https://git.openjdk.org/jdk/pull/18800/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18800&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8330339
  Stats: 18 lines in 1 file changed: 9 ins; 9 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/18800.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18800/head:pull/18800

PR: https://git.openjdk.org/jdk/pull/18800