Defer destroy activity if it is in transition.

When request finish translucent activity, because the below activity
could be visible, core can destroy the activity immediately. And if
the activity is participant a transition, the client can suddely
become invisible due to onStop called.

Bug: 318877215
Bug: 323129567
Test: follow issue description, monitor the top CustomTabActivity
won't be destroyed right after clear-task.

Change-Id: Ib8548dfa3d5b5ea4e91ba95032c56d1358ff0bf1