RFR: 8283683: Make ThreadLocalRandom a final class

[Jaikiran Pai]

Can I please get a review of this change which marks the `ThreadLocalRandom` class as `final`? Related JBS issue https://bugs.openjdk.java.net/browse/JDK-8283683.

A CSR has been filed too https://bugs.openjdk.java.net/browse/JDK-8283688.

tier1, tier2 and tier3 tests have been run with this change and no related failures have been noticed.

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Commit messages:
 - 8283683: Make ThreadLocalRandom a final class

Changes: https://git.openjdk.java.net/jdk/pull/7958/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7958&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8283683
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7958.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7958/head:pull/7958

PR: https://git.openjdk.java.net/jdk/pull/7958