Polish the app reveal animation.

This CL polishes the app reveal animation after a cold start. It does so
by:

1. Fading out the icon during 233ms before revealing the app, as spec'ed
   in the direct [1].
2. Animating the reveal gradient opacity from 0 too 100% (instead of
   starting at 100% opacity directly).

See b/184830058#comment7 and #comment13 for before/after videos.

[1] https://direct.googleplex.com/#/spec/241550001

Bug: 184830058
Test: atest SplashscreenTests StartingSurfaceDrawerTests
Change-Id: Ie4561887a259eec398f8f347050f27a094026945