Fix mNoAnimActivities

Since we split tasks into separate stacks, mNoAnimActivites from
other tasks/stacks were pretty much never cleared, leading to
skipped animations in the following cases:
- Enter/exit multi-window, relaunch anything, animation was
always skipped.
- Reopen PIP activity from notification shade

Now, we make this a global list which we immediately clear
after using it.

Test: go/wm-smoke
Test: Enter/exit multi-window, relaunch
Bug: 64674361
Change-Id: Ic36d278b263d6b3c6a7ec7416c8b94c4eed8e440