RFR: 8297750: Unnecessary Vector usage in IIORegistry

[Andrey Turbanov]

Field `javax.imageio.spi.IIORegistry#initialCategories` is modified only in `static {}` block, which makes it effectively final. Instead of legacy synchronized `Vector` we can use non-threadsafe `ArrayList` here.

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Commit messages:
 - 8297750: Unnecessary Vector usage in IIORegistry
 - [PATCH] Unnecessary Vector usage in javax.imageio.spi.IIORegistry

Changes: https://git.openjdk.org/jdk/pull/11379/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11379&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8297750
  Stats: 17 lines in 1 file changed: 1 ins; 7 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/11379.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11379/head:pull/11379

PR: https://git.openjdk.org/jdk/pull/11379