On disabled app pairs, set color tint instead of changing alpha

This CL changes the way disabled app pair icons are drawn: instead of setting alpha, it sets a color filter to indicate disabled status. This brings it in line with other apps icons and V's design specs.

Fixes: 326665140
Flag: ACONFIG com.android.wm.shell.enable_app_pairs TRUNKFOOD
Test: Manual
Change-Id: Ifc8ac694f321a6b28996ba9a42860dfc419d5901