https://*.com/questions/3609174/android-insert-text-into-edittext-at-current-position
Cpt.Ohlund gave me the right hint. I solved it, now, partly with using EditText.getSelectionStart(), but I realized that you can also replace the selected text with the same expression and you don't need String.subString() for that.
int start = Math.max(myEditText.getSelectionStart(), 0);
int end = Math.max(myEditText.getSelectionEnd(), 0);
myEditText.getText().replace(Math.min(start, end), Math.max(start, end),
textToInsert, 0, textToInsert.length());This works for both, inserting a text at the current position and replacing whatever text is selected by the user. The Math.max() is necessary in the first and second line because, if there is no selection or cursor in the EditText, getSelectionStart() and getSelectionEnd() will both return -1. The Math.min() and Math.max() in the third line is necessary because the user could have selected the text backwards and thus start would have a higher value than end which is not allowed for Editable.replace().