Load the split divider veil icons on a boosted background thread

- Re-land ag/27118118 (trigger loading of split veil icons at touch
  down vs after moving past a threashold), and move loading of the
  split resize veil icons to the background thread (and boost during
  icon load to ensure we get them with foreground priority)

Bug: 370895085
Bug: 351138552
Flag: EXEMPT bugfix
Test: Splitscreen smoke test & take perfetto trace when moving the divider
Change-Id: Ia416cea8709bdeac5b47a4efda785079804a312e