попробуйте это,
editText.addTextChangedListener(new TextWatcher() {
@Override
public void beforeTextChanged(CharSequence charSequence, int i, int i1, int i2) {
}
@Override
public void onTextChanged(CharSequence charSequence, int i, int i1, int i2) {
if (charSequence.toString().equalsIgnoreCase(" ")) {
editText.setText(charSequence.toString().replace(" ", "%"));
}
}
@Override
public void afterTextChanged(Editable editable) {
}
});