RFR: 8349105: Pagination: exception initializing in a background thread

[Andy Goryachev]

## Root Cause
Animation gets started in a background thread, which causes the animation handler to run in the FX application thread, thus creating simultaneous access to the control's fields (list of children in this case).

## Solution
Postpone the animation unless running in the FX application thread.  There is no functional difference if the component is created/used in the FX application thread.

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Commit messages:
 - postpone animation

Changes: https://git.openjdk.org/jfx/pull/1698/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1698&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8349105
  Stats: 11 lines in 2 files changed: 4 ins; 5 del; 2 mod
  Patch: https://git.openjdk.org/jfx/pull/1698.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1698/head:pull/1698

PR: https://git.openjdk.org/jfx/pull/1698