Update the gradient search to go deeper down the tree of drawables.

Before we only inspected the top level of drawables inside a
LayerDrawable, which didn't work in case this was in turn one of the
intermediate categories (in the related bug, an InsetsDrawable).

This new approach allows us to continue going down the tree to find any
GradientDrawable.

Bug: 321654048
Flag: NA
Test: see videos in the bug
Change-Id: Icb6f4b001c66bd78c3eb8167cb163d4823e4258f