Force new fading elements to fade from 0 alpha

This CL makes sure that new elements that were never drawn in any scene
and that appear by fading always start at alpha = 0f. This ensures that
elements never appear instantly even if the transition making them
appear on screen start at progress >> 0f.

Bug: 290930950
Test: ElementTest
Flag: com.android.systemui.scene_container
Change-Id: I7e5f70fd9657d5b8c2dd29881828b973a7ac1e4d