RFR: JDK-8224260: ChangeListener not triggered when adding a new listener in invalidated method

[John Hendrikx]

Fixes three issues in ExpressionHelper:

- Current Value was not retained when changing from SingleChange to Generic, this can lead to missed changes
- Current Value was not retained when changing from Generic to SingleChange, this can lead to missed changes
- Current Value was not cleared when last change listener was removed in Generic variant, resulting in an older value being referenced and not becoming eligible for GC until either a ChangeListener is added again, or sufficient InvalidationListeners are removed to switch to the SingleInvalidation implementation...

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Commit messages:
 - Fix current value handling in ExpressionHelper

Changes: https://git.openjdk.org/jfx/pull/1078/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1078&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8224260
  Stats: 114 lines in 2 files changed: 90 ins; 10 del; 14 mod
  Patch: https://git.openjdk.org/jfx/pull/1078.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1078/head:pull/1078

PR: https://git.openjdk.org/jfx/pull/1078