Minimize the scope of some string consts

Help prevent "duplicate symbol" linker errors when this code is included more than once in an app (because one or more included libraries also includes this code).

Note that this set of changes does not solve the entire problem. There are several other string consts which are declared in the `.h` file. So if an app and one of its libraries both include a copy of this code, then one or both of them will need to rename those string consts to prevent linker collisions.

If there's not a clever technical fix (none has yet occurred to me), then another thought would be to modify the names of all these strings consts. For example `OnePasswordAppExtensionURLStringKey` -> `YourAppPrefixOnePasswordAppExtensionURLStringKey`, so as to at least suggest what each developer should do. (Use of preprocessor string concatenation, to improve DRYness, is also left as an exercise for the reader.)