Accumulating tab presses during animations

Also introducing these animations. Will need to talk to UX regarding
these animations' properties. So far, I'm keeping a growing list of
questions to UX.

Bug: 73090995
Test: Press, Alt-tab, tab with and without shift, press Alt-Tab on
already open Overview.

Change-Id: Ib7250349acc528d14d8702f60b0f205612205478