RFR: 8373335: Serial: Clean up SerialHeap members by access specifies

[Albert Mingkun Yang]

Trivial moving member of `SerialHeap` so that all `private` fields/methods are altogether (at the top).
 
Test: tier1

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Commit messages:
 - sgc-private

Changes: https://git.openjdk.org/jdk/pull/28718/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28718&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8373335
  Stats: 40 lines in 2 files changed: 18 ins; 21 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/28718.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28718/head:pull/28718

PR: https://git.openjdk.org/jdk/pull/28718