RFR: 8300866: Declare some classes final in java.io

[Per Minborg]

This PR proposes declaring some internal classes in java.io as `final`, explicitly indicating that they are not intended for sub-classing. 

If you think declaring classes final is just "static noice", please see http://minborgsjavapot.blogspot.com/2021/12/why-general-inheritance-is-flawed-and.html first and then come back and comment on the issue.

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

Commit messages:
 - Declare classes as final

Changes: https://git.openjdk.org/jdk/pull/12141/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12141&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8300866
  Stats: 11 lines in 5 files changed: 0 ins; 0 del; 11 mod
  Patch: https://git.openjdk.org/jdk/pull/12141.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12141/head:pull/12141

PR: https://git.openjdk.org/jdk/pull/12141