RFR: 8298567: Make field in RandomAccessFile final [v2]

[Per Minborg]

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

Per Minborg has updated the pull request incrementally with one additional commit since the last revision:

  Make the field rw final

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/11632/files
  - new: https://git.openjdk.org/jdk/pull/11632/files/aa682717..20c9d49a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11632&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11632&range=00-01

  Stats: 21 lines in 1 file changed: 12 ins; 6 del; 3 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