Update Window#setResizingCaptionDrawable

New window decorations are drawn in SystemUI, so their responsiveness
isn't limited by app's responsiveness like old captions. Therefore,
there is no need to expose an API to apps to provide a background
drawable to improve UX while resizing.

Apps can choose to draw in caption areas, which is a good replacement of
old drawables.

Bug: 333724879
Test: Builds.
Change-Id: I44c62a591ed5ce962cedd81697234d3a18fdb2e3