[Autofill Assistant] Refactored the OverlayImage to take an arbitrary Drawable as input.

Before this CL, overlay images were always retrieved from a target URL.
This CL allows the backend to specify the image from a variety of sources, as used in other places of the system. This will make overlay images more versatile.
The old way of fetching images from URLs is still supported by specifying a BitmapDrawable as input.

Bug: b/168023114
Change-Id: I994dc10cda30e04f77cb8acbe10563575417d1b3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2410067
Commit-Queue: Rahul Goyal <goyalrahul@google.com>
Reviewed-by: Clemens Arbesser <arbesser@google.com>
Reviewed-by: Marian Fechete <marianfe@google.com>
Cr-Commit-Position: refs/heads/master@{#816091}