RFR: 8365711: Declare menuBarHeight and hotTrackingOn private

[Alexey Ivanov]

Since `WindowsMenuUI` is a final class after PR #24170 for [JDK-8352638](https://bugs.openjdk.org/browse/JDK-8352638)), it cannot be extended, therefore the `protected` modifier doesn't make sense.

Mark both `menuBarHeight` and `hotTrackingOn` as `private` fields.

-------------

Commit messages:
 - 8365711: Declare menuBarHeight and hotTrackingOn private

Changes: https://git.openjdk.org/jdk/pull/26829/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26829&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8365711
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/26829.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26829/head:pull/26829

PR: https://git.openjdk.org/jdk/pull/26829