RFR: 8283214: [macos] Screen magnifier does not show the magnified text for JcomboBox

[Abhishek Kumar]

The issue exist only for non-editable combobox and the root cause is accessible object is not created due to incorrect index returned from component class which results in no a11y API invoked.

Proposed solution is to return the correct accessible child from getAccessibleChild method which is AquaComboBoxButton (arrowButton) instance and that results in invoking the a11y APIs to return the current selected item in combobox. 

Further when the application comes up first time the accessible name is not set for current displayed item in JCombobox that is handled in AquaComboBoxButton which will take care for the current selected item as well as if user modifies the selection by drop-down list.

This fix results in few JCK test failure and that may require modification. I have filed an issue against JCK ([JCK-7319423](https://bugs.openjdk.org/browse/JCK-7319423)).  

CI link is posted in JBS.

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Commit messages:
 - JComboBox selected item  screen magnifier fix

Changes: https://git.openjdk.org/jdk/pull/14497/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14497&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8283214
  Stats: 124 lines in 3 files changed: 103 ins; 5 del; 16 mod
  Patch: https://git.openjdk.org/jdk/pull/14497.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14497/head:pull/14497

PR: https://git.openjdk.org/jdk/pull/14497