[4/n] Add Apply Button to grid preview page

1. Preview a different grid will enable "Apply" Button
2. Tap on "Apply" button will reload workspace and show toast, and disable "Apply" button

Flag: wallpaper_picker_grid_apply_button
Bug: 294866224
Test: enable wallpaper_picker_grid_apply_button flag and attach video in bug, disable wallpaper_picker_grid_apply_button flag and verified no UI change
Change-Id: I63ad75cec947a3437622cc9c97dc5e9b63e46ff1