I don't think a mask formatter is what you want for this because you aren't really formatting a string here. If you just want to take a string and set characters at certain positions you should formulate the string yourself.
String text = "";
for (int i = 0; i < WORD.length(); i++) {
if (String.valueOf(word.charAt(i)).equals(letter)) {
text += letter + " ";
} else {
text += "? ";
}
}
input.setText(text);
Also a style note: all caps is for identifiers of static variables.