RFR: 8254956: [REDO] Memoryleak: Closed focused Stages are not collected with Monocle

[Jose Pereda]

This PR fixes a memory leak, as a follow-up of [JDK-8241840](https://bugs.openjdk.java.net/browse/JDK-8241840).

The tests pass on desktop (Linux/macOS/Windows) and there is no regression on Android (see [JDK-8254605](https://bugs.openjdk.java.net/browse/JDK-8254605)).

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Commit messages:
 - Enable test back and include required sw pipeline
 - Nullify focusedWindow when window is closed and add null checks

Changes: https://git.openjdk.java.net/jfx/pull/677/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=677&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254956
  Stats: 20 lines in 4 files changed: 11 ins; 6 del; 3 mod
  Patch: https://git.openjdk.java.net/jfx/pull/677.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/677/head:pull/677

PR: https://git.openjdk.java.net/jfx/pull/677