Lazy initialize split divider MotionSpec

* This may be a culprit for non-split related perf issues,
so we're trying to see if moving this to divider interaction helps.
* There's a chance this could then regress on split divider CUJs, but
then at least that's localized.

Bug: 425461336
Flag: com.android.wm.shell.enable_flexible_two_app_split
Test: Perf tests will tell! Works fine otherwise
Change-Id: If208f24ee281bb1e59e7b267968990358e88de9e