RFR: 8309374: Accessibility Focus Rectangle on ListItem is not drawn when ListView is shown for first time

[Ambarish Rapte]

This is accessibility specific fix.

**Issue**: When a ListView is shown for first time then accessibility focus rectangle is not drawn around the focused ListIem

**Cause:** 
   The ListView takes a little time to create it's skin(ListViewSkin) and the skins for ListItems(ListCellSkin)
   If the Accessibility client application requests focused item before ListView is completely ready then JavaFX return null.

**Fix**: Send a focus item change notification to accessibility client application after the ListIteam is ready

**Verification:**
- On Windows machine, launch Narrator. Issue is observed only with Narrator
- Execute the [program](https://bugs.openjdk.org/secure/attachment/104180/Main.java) attached to the JBS issue [JDK-8309374](https://bugs.openjdk.org/browse/JDK-8309374)
     Move through the TextFields and press `Alt+down`, observe that focus rectangle is drawn correctly.
     Once the ListView is showing Press and hold `Up/Down` or `Ctrl + Up/Down` keys, observe that focus rectangle is always drawn.

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Commit messages:
 - a11y ListView focused item fix

Changes: https://git.openjdk.org/jfx/pull/1363/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1363&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8309374
  Stats: 31 lines in 1 file changed: 31 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jfx/pull/1363.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1363/head:pull/1363

PR: https://git.openjdk.org/jfx/pull/1363