Load and draw bitmap only once with ImageWallpaper

Given that ImageWallpaper uses surface.SetFixedSize with the dimensions
of the image, and that WallpaperController moves the without redrawing
the bitmap (e.g. in case of rotation/unfold), the bitmap only needs to
be drawn once (in theory).

Bug: 288582051
Bug: 265018814
Test: manual on foldable: fold, unfold, rotate, with one or two
ImageWallpaper engines
Test: atest ImageWallpaperTest

Change-Id: I1a61c5d30c5f4d6abff4eac76195b74dfb18fe6f