Align the duration of task exit animation to enter animation

Now they are both 500ms. Otherwise if the duration of exit animation
is longer, it may cover the entering task.

Bug: 201998178
Test: Back from a task in a different orientation, the exiting
      task should not flash on screen.

Merged-In: Iefa134e4217ba35d33801225368b7a5df60d932c
Change-Id: Iefa134e4217ba35d33801225368b7a5df60d932c