Remove shadows from app launch animation

The Material You style proposes a flat model without shadows.

This will also help us reduce overdraw because shadows are drawn in
client composition, causing an almost full screen redraw, given that
shadows have the size of the window, which is not usually marked as
opaque.

Bug: 188420494
Test: manual
Change-Id: I5073788a247ea9084b8e3399d4ca8618c21b8d6b