RFR: 8301604: Replace Collections.unmodifiableList with List.of

[Glavo]

`List.of` is cleaner, and can slightly reduce the memory footprint for lists of one or two elements.

Because `List.of` can only store non-null elements, I have only replaced a few usage.

Can someone open an Issue on JBS for me?

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Commit messages:
 - cleanup

Changes: https://git.openjdk.org/jfx/pull/1012/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1012&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8301604
  Stats: 18 lines in 3 files changed: 0 ins; 9 del; 9 mod
  Patch: https://git.openjdk.org/jfx/pull/1012.diff
  Fetch: git fetch https://git.openjdk.org/jfx pull/1012/head:pull/1012

PR: https://git.openjdk.org/jfx/pull/1012