Add unocclude animation.

This is a simple downward translate/alpha animation to match
the shade's current unocclude animation. We use this rather
than activity lauch animator since we're not launching from
a view/from the power button, so it's not needed.

Bug: 202963722
Test: unocclude camera
Test: unocclude home controls
Test: unocclude google maps navigation
Change-Id: Id62fcb75d9215d8b12d06fb8c2e7e6e884e457b9