Release WallpaperService SurfaceControl directly

The SurfaceControl is created with the engine and filled in later after
the first time attachging. It can be released as soon as the engine is detached, since wallpaper engines are never reused after detaching.

Test: atest WallpaperManagerTest # after which, check dumpsys surfaceflinger for leftover offscreen wallpaper layers
Test: adb shell su root kill -10 `adb shell pidof system_server` # and then take a heap dump to ensure the IWindow is also gone.
Bug: 261358202
Change-Id: I86f0a3ee9b8272d61bbc8f9c5f80ee3f0ddac96f