RFR: 8298567: Make field in RandomAccessFile final

[Per Minborg]

This PR proposes making a field in `RandomAccessFile` final. Also, it modernises a switch statement.

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Commit messages:
 - Reclare field final and modernize switch

Changes: https://git.openjdk.org/jdk/pull/11632/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11632&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8298567
  Stats: 15 lines in 1 file changed: 0 ins; 5 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/11632.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11632/head:pull/11632

PR: https://git.openjdk.org/jdk/pull/11632