Don't give up on the launch animation if there is no task info.

We only use the task info for background color. If the task info is
missing but the mode is correct, it means we are in an
activity->activity transition within the same task. We can get the
background color directly from the window.

Bug: 320961634
Flag: NA
Test: see before and after videos in b/320963010
Change-Id: I80b2ec01ca574139a3fafabdc68648f555919ba9