RFR: 8284105: Update security libraries to use sealed classes

[Sean Mullan]

Please review these changes to update the security libraries to use sealed classes. See JEP 409 for more details on sealed classes.

No CSR is required as all the changes are to internal classes. A few classes that did not have subclasses were simply marked final instead of sealed.

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

Commit messages:
 - Update security libraries to use sealed classes.

Changes: https://git.openjdk.java.net/jdk/pull/8165/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8165&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8284105
  Stats: 50 lines in 20 files changed: 8 ins; 0 del; 42 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8165.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8165/head:pull/8165

PR: https://git.openjdk.java.net/jdk/pull/8165