Fix osk.defaultKeyOutput to handle characters beyond U_FFFF

`String.fromCharCode()` has a limitation to Unicode characters in the BMP. Change to `String.kmwFromCharCode()` to handle entire Unicode range.

Also clarified how default key codepoints are processed.

KeyboardHarness changes:
- Change text area font on keyboard change
- Added U_10481 to test longpress keyboard.
- Remove test tchad keyboard