[framework] Modify scalePreview in WallpaperService

Modified the implementation of scalePreview to call setFixedSize instead

This function is used by the wallpaper picker to resolve the scaling
issue.

A TODO b/262750854 is created to later have a holistic fix.

Bug: 204844104
Bug: 240291366
Test: Manuel
Change-Id: Idd45eaac34db69d7e2bdd5450678b7d7e86d4629