Initialize split resize veils on touch down

- Upfront the work of creating the resize veil and loading the veil app
  icons on DOWN instead of waiting for the user to pass the touch slop.
  This allows the initialization to take place while the divider is not
  yet moving, and prevents jank from loading icons/other binder calls
  mid-gesture (at the cost of doing a bit more work on DOWN).

Bug: 337517681
Test: Take a perfetto trace and verify that the heavy work happens on
      DOWN outside of the resize sections
Change-Id: Ieb21ef0b43fd610f1aec8a1cc56b14ae976483a1