Grant set wallpaper dim amount permission to Wallpaper Picker (2/5)

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: I37f42df7fd374973ae95b106c37b324d52f213f7