chore(ios): add iPad detection robustness

> Pretty sure we already do this [#8042] for the keyboard's WebView in Swift-land too, but I'm not quickly finding which PR that occurred in. `git blame` can probably find it if we care enough, though.

And... we don't? I'm not seeing it via repo-wide string search. Thought we at least _meant_ to add it as an extra layer at some point... that may be worth a small PR, to add robustness.

Of course, that means the Web spoof-detection code I referenced before has been doing all the "heavy lifting" for iOS tablets for years now.

I'd put the corresponding changes in this method: https://github.com/keymanapp/keyman/blob/f12cd852a6996ea4586a5a931f5e641ad6c1029c/ios/engine/KMEI/KeymanEngine/Classes/Keyboard/KeymanWebViewController.swift#L119

_Originally posted by @jahorton in https://github.com/keymanapp/keyman/issues/8042#issuecomment-1397815398_