Add delay to notifications before they can be dismissed.

Test: Run jank tests. Create notifications with a delay, then attempt
to swipe them away as soon as they are perceivable to the user.
Swiping during the animation should not result in notification dismissal,
but swiping after the animation completes should.
Fixes: 70293267
Change-Id: Ibca02666255693255185b8bd326cc97d8c574b2e