Keep disabled FastBitmapDrawables disabled while fast scrolling.

Previously, they were animating to be colored because they were
set to have a FAST_SCROLL_UNLHIGHLIGHTED state. Now they retain
their disabled color when changing fast scroll states.

Specfically, we remove the DISABLED state and instead make it a
property of the FastBitmapDrawable.

Bug: 32642959
Change-Id: I6cb2da134a550c267eebfc756eff8c91a33f028c