Grant set wallpaper dim amount permission to Wallpaper Picker (4/5)

Grant permission needed for the new WallpaperManager method
getWallpaperColors, to prepare for enabling color preview in the
multi-crop UI.

Flag: ACONFIG com.android.wallpaper.multi_crop_preview_ui_flag DEVELOPMENT
Bug: 315856338
Test: manually verified
Change-Id: Ied82df73e7f3d3573b01ecf7335824c37cfd2662