RFR: 8300084: AquaFileChooserUI.getDefaultButton returns null [v3]

[Tejesh R]

> In Aqua look and feel, `getDefaultButton()` of `JFileChooser` returns null. The fix overrides the method from `FileChooserUI` to return the `ApproveButton` of `FileChooser` which is similar to other L&F. 
> The test is run for multiple iterations in CI machine.

Tejesh R has updated the pull request incrementally with one additional commit since the last revision:

  Removed the test - Updating #11901 test

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Changes:
  - all: https://git.openjdk.org/jdk/pull/12008/files
  - new: https://git.openjdk.org/jdk/pull/12008/files/81d2cda2..603fd66a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12008&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12008&range=01-02

  Stats: 54 lines in 1 file changed: 0 ins; 54 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/12008.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12008/head:pull/12008

PR: https://git.openjdk.org/jdk/pull/12008