schemas: Update touchscreen/tablet mapped output key

Even though the setting doesn't change in essence, it has different semantics wrt its "default" value as it will no longer be maintained by g-s-d and honored by mutter, but handled entirely by mutter.

This means the key won't hold the results from g-s-d mapping heuristics but is expected to be NULL if heuristics from mutter should apply. Generally, only opaque tablets will require this value to be overwritten, but the key will be honored on all devices in case heuristics go wrong.

Related: mutter!133 (merged)