Fix off-center rendering on some devices

[powerje]

Summary

• Replaces hasCenteredImages boolean flag with lastCanvasWidth/lastCanvasHeight dimension tracking
• Re-centers sprites whenever canvas dimensions actually change, rather than relying solely on surfaceChanged callback
• Two int comparisons per frame; setCenter only called when dimensions differ

Fixes #10

Test plan

• Verify wallpaper stays centered after rotation
• Verify wallpaper stays centered during home screen page swipes
• Verify wallpaper stays centered in split-screen / multi-window mode
• Verify no visible performance regression (animation smoothness, battery)