These all are ways:
String imageUri = "drawable://" + R.drawable.image;
Other ways I tested
Uri path = Uri.parse("android.resource://com.segf4ult.test/" + R.drawable.icon);
Uri otherPath = Uri.parse("android.resource://com.segf4ult.test/drawable/icon");
String path = path.toString();
String path = otherPath .toString();