RFR: 8300864: Declare some fields in java.io as final

[Per Minborg]

Some of the fields in java.io can be declared as final.

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Commit messages:
 - Revert final class declaration
 - Declare fields final

Changes: https://git.openjdk.org/jdk/pull/12140/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12140&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8300864
  Stats: 117 lines in 8 files changed: 5 ins; 5 del; 107 mod
  Patch: https://git.openjdk.org/jdk/pull/12140.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12140/head:pull/12140

PR: https://git.openjdk.org/jdk/pull/12140