Check okToAnimate for TaskFragment change transition

1. Similar to ActivityRecord#setVisibility, check okToAnimate before
preparing transition.
2. Similar to ag/16063813, disable transition animation when screen
blocker is not removed.

Bug: 204035526
Test: atest WmTests:TaskFragmentTest
      #testNotOkToAnimate_doNotStartChangeTransition
Change-Id: I55486b456e4fd0a93b74faedcdc8a6f01bcc3737