[grouping info] Fix potenial "Marked in-app by client" false positives

Right now, if a frame gets to the grouping algorithm with `in_app` already set, we mark it in the grouping info section as "Marked in-app by the client." This seems to be showing up much more regularly than expected, though, so it's possible that there's some other way it's getting set (relay? symbolicator?), making our frame hint inaccurate. Let's investigate to see if this is the case, and if so, try to fix it.