chore(android): preference key names should not be resource strings

> afaict, the preference key name has always been a resource string back in Keyman for Android 2.8

Can we do a chore to pull it into a constant? It's silly to be grabbing resources in order to just get a fixed string name. It's not translatable, and it should never change.

_Originally posted by @mcdurdin in https://github.com/keymanapp/keyman/issues/13578#issuecomment-2764886503_