Make power menu animaiton smoother

This animation was being mapped directly to the alpha of the dialog,
which makes it a bit stiff. I'm now using a physics based spring
animation, sharing logic with the shade.

Fixes: 149792636
Test: atest NotificationShadeDepthControllerTest
Change-Id: Iad8835b8040d1aedc13f0e323fc747e4ab4813e9