Clear task when opening WallpaperPicker

-> Each session can reasonably be considered a fresh start
-> Also, works around b/79507056

Change-Id: Iac1ae59cd0c743d71c91a34ba32a0fef51548f74

src/com/android/launcher3/views/OptionsPopupView.java

@@ -184,6 +184,7 @@ public class OptionsPopupView extends ArrowPopup
         Intent intent = new Intent(Intent.ACTION_SET_WALLPAPER)
                 .putExtra(EXTRA_WALLPAPER_OFFSET,
                         launcher.getWorkspace().getWallpaperOffsetForCenterPage());
+        intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TASK);
 
         String pickerPackage = launcher.getString(R.string.wallpaper_picker_package);
         if (!TextUtils.isEmpty(pickerPackage)) {