Fix boot animation alpha multiplication computations

Previously, this was resulting in the output color roughtly as

  output_n = color_n * mask_n^2 * (mask_0 + mask_1 + mask_2 + mask_3)

where n is a color channel between 0 and 3.

This caused extra aliasing around edges, since the alpha fades too
quickly along an edge.

Bug: 239632466
Bug: 258246539
Test: verify pixel values for boot animation match mask on watch + phone
Test: modify uFade value, verify pixel colors match
Change-Id: I86742a181576e684aa698a190a8766f4223de799