Fix Steam Input shortcut app ID key

[sean0407]

Summary

- Write Steam Input settings under the signed shortcut AppID
+ Write settings under the unsigned AppID Steam uses
+ Remove a stale signed key owned by Steam ROM Manager

Evidence

• Before: Steam ROM Manager writes UseSteamControllerConfig using the signed key (-1747299846). Issue #891 reports Steam stores the manually applied setting under the unsigned key (2547667450).
• After: the writer uses shortenAppId for the unsigned key and clears an old signed key only when it is tagged as Steam ROM Manager data.
• Checked the reported ID conversion with Node.js: 10956423566568697856 maps to unsigned 2547667450 and signed -1747299846. git diff --check passed; the patch policy accepted 1 file and 11 changed lines.

Merge Danger

Door: two-way

Blast Radius: small

The change affects per-shortcut Steam Input settings in localconfig.vdf.