Use intercept layer to prevent touches during animation instead of defocusing

- Instead of adjusting the focusability of the root tasks (which has
  other core side-effects), we can abstract out the intercept layer logic
  used by OffscreenTouchZone to also block touches on the top split
  root for the duration of the swap animation

Fixes: 429058310
Flag: EXEMPT bugfix
Test: Manual, ensure we still ignore touches during swap animation,
      and test with/without bubbled task
Change-Id: Id4a5c5d05cb66df19cc01b62aeaedab4d26a5907