Continue to remove code around legacy captions

The most prominent removal in this CL is the removal of
BackdropFrameRenderer. No code would ever create BackdropFrameRenderer
even without this CL, so we can safely remove it. There is a piece of
code in libhwui that was introduced with it, but I decide to clean it up
after this CL goes in just in case some other code I don't know relies
on that.

This CL also removed a bunch of resources tied to the legacy caption.

It also removed the implementation of
android.view.Window#setResizingCaptionDrawable() which doesn't do
anything with captions in WM shell. We should optimize our captions well
enough to be responsive enough for fluid resizing, so we don't need the
app to provide a background drawable to alleviate janky UX.

It also removed mResizingBackgroundDrawable. Unlike other removed
drawables, mResizingBackgroundDrawable was also used to calculate
background paddings. Changed the code that calculates background
paddings to avoid using it as well. The new logic doesn't enforce
non-translucent background anymore because the old logic only changes
ColorDrawable, which doesn't have any paddings with any colors.
Therefore the new logic is equivalent to the old one with regard to
background paddings.

Bug: 333724879
Test: Presubmit tests.
Change-Id: Ie98c9226935d22cfa4d98df8ade3f604f57a10d4