Updated GradientDrawable setColor(ColorStateList) implementation to

match corresponding documentation to fall back to
setColor(Color.TRANSPARENT) in the event that null is passed as the
ColorStateList parameter

Refactored implementation to directly call setColor(Color.TRANSPARENT)
if setColor(null) is invoked, otherwise configure the solid colors and
the fill Paint color to the ColorStateList and resolved color for the
given state

Bug: 64951083
Test: Compiled, flashed an updated build of master onto taimen and
re-ran tests with updated implementation in GradientDrawableTest

Change-Id: Ia7e9a995fc78e49d601931ca6af81470d90f8439