RFR: 8283417: Update java.nio buffers to use sealed classes

[Brian Burkhalter]

Seal subclassed buffer classes and make non-subclassed buffer classes final.

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Commit messages:
 - 8283417: Update java.nio buffers to use sealed classes

Changes: https://git.openjdk.java.net/jdk/pull/7907/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7907&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8283417
  Stats: 45 lines in 7 files changed: 37 ins; 0 del; 8 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7907.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7907/head:pull/7907

PR: https://git.openjdk.java.net/jdk/pull/7907