Move deprecated images to images-unused

This includes:
activity-indicator-32x32.png
back-button.png
dashboard-mask-bottom.png
dashboard-mask-top.png
dashboard-scroll-fade.png
dock-item-shadow-tile.png
empty-launcher.png
fade-arrow-down.png
loading-card-scrim.png
menu-selection-gradient-last.png
meta-move.png
overlay-banner-bg.png
popup-scrollfade-bottom.png
quick_launch_highlight.png
reorder-ripple.png
search-bottom-fade.png
search-pill-no-icon.png
transient-alart-bg.png