RFR: 8283705: Make javax.sound.midi.Track a final class

[Phil Race]

This is the straggler from several PRs which were around making JDK classes sealed.
There's nothing to be sealed here, but the same query pointed out that this class has
no public or protected constructor (as well as no sub-classes) and so can be made final

CSR for review here https://bugs.openjdk.java.net/browse/JDK-8285978

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Commit messages:
 - 8283705

Changes: https://git.openjdk.java.net/jdk/pull/8492/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8492&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8283705
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8492.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8492/head:pull/8492

PR: https://git.openjdk.java.net/jdk/pull/8492