Fix task animations

Sometimes apps are using trampoline activities in the current
task, and then open a new task from that trampoline activity,
which means that the first transition was ACTIVITY_OPEN and that's
what we take at end.

To fix this, we let task animations always override activity
animations.

Test: go/wm-smoke
Test: Open a Docs document from Drive. Open a Chrome link from
Maps.
Fixes: 72710883

Change-Id: I72b6901fdc21514ffca4a47e433d66ea9d3cdb32