Allow downstream changes to OSK layouts

Quoting the main commit:

We had various requests to improve existing OSK layouts, but
haven't accepted them so far as any changes would be overridden
when regenerating the layouts.

However as the upstream layouts at http://www.unicode.org are
extremely slow to update(*), we shouldn't block all improvements.

So instead of letting the update script override all existing
layouts, just make it import new layouts.

(*) not their fault, as the android layouts are a downstream to Google