RFR: 8286663: Resolve IDE warnings in WTrayIconPeer and SystemTray [v3]

[Alexey Ivanov]

> **WTrayIconPeer**: removed duplicate call to `popupParent.dispose()` that might cause NPE (it looks `popupParent` cannot be `null`); organised imports.
> 
> **SystemTray**: removed redundant initialisers; replaced sized array with empty array in `toArray` call; dropped `newValue != null` chained with `equals`.

Alexey Ivanov has updated the pull request incrementally with one additional commit since the last revision:

  Use method reference in setSystemTrayAccessor instead of anonymous class

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8850/files
  - new: https://git.openjdk.java.net/jdk/pull/8850/files/a2f8c952..1f47fe4f

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8850&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8850&range=01-02

  Stats: 9 lines in 1 file changed: 0 ins; 8 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8850.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8850/head:pull/8850

PR: https://git.openjdk.java.net/jdk/pull/8850