Fix issue with task snapshots being taken too late

- Currently, a display change while recents transition is playing will
  result in the display change being applied prior to the recents
  animation playing, which means that the transition-end snapshots are
  taken in the wrong display configuration
- Instead, When a display change is requested mid-recents transition,
  cancel the current transition and record the snapshots in place
  (this matchs legacy recents animation behavior)
- Separately, if the snapshot is requested & recorded mid transition,
  then we can ignore the post-transition screenshot (which would be
  wrong in the display change case, or just extraneous even in the normal
  case where launcher has requested a screenshot itself already).
  We do this by comparing the time that the last snapshot was taken
  with the start of the transition.

Fixes: 278189494
Test: Go into Overview and rotate the device and observe the task views
Change-Id: I985cd9c852a326027ae3ab4f7ffe837431d1c654