RFR: 8303690: Prefer ArrayList to LinkedList in com.sun.jmx.mbeanserver.Introspector

[Andrey Turbanov]

`LinkedList` is used as value in `com.sun.jmx.mbeanserver.Introspector.SimpleIntrospector#cache`
It's created, filled (with `add`) and then iterated. No removes from the head or something like this. `ArrayList` should be preferred as more efficient and widely used collection.
Also I've done some related code cleaned:
1. removed redundand `if` from `SoftReference` value check
2. fixed a typo in javadoc

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Commit messages:
 - [PATCH] Prefer ArrayList to LinkedList in Introspector

Changes: https://git.openjdk.org/jdk/pull/12839/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12839&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303690
  Stats: 8 lines in 1 file changed: 1 ins; 3 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/12839.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12839/head:pull/12839

PR: https://git.openjdk.org/jdk/pull/12839