Fade in shelf icons (replaces y-translation/scale/color-inversion)

Fade based on notification-to-shelf transition amount.
Always show notification row icon. Remove "setShelfIconVisible"

Bug: 178479515
Test: expand shade and scroll (LTR/RTL)
   => notification row always shows icon
   => shelf icons fade in and out with no yTranslation

Change-Id: I077f4fcfd25198184e0bdb8f1dcc37710f5e57b1