RFR: 8314820: Unify Screen adapter ordinal logic

[Marius Hanl]

This is a small improvement that was noted back in https://github.com/openjdk/jfx/pull/1200#issuecomment-1671954361 by @mstr2, and was on my list since then.

The idea is to have only one place for assigning `adapter ordinals` – within `Screen`. The corresponding setter `setAdapterOrdinal` can therefore be made private. This improves the separation of responsibilities a little. :-)

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Commit messages:
 - 8314820: Unify Screen adapter ordinal logic

Changes: https://git.openjdk.org/jfx/pull/2009/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=2009&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8314820
  Stats: 24 lines in 3 files changed: 9 ins; 9 del; 6 mod
  Patch: https://git.openjdk.org/jfx/pull/2009.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/2009/head:pull/2009

PR: https://git.openjdk.org/jfx/pull/2009