Don't set the id; the following will do what you want:
ContentResolver cr = getContentResolver();
ContentValues values = new ContentValues();
values.put (Events.CALENDAR_ID, Long.toString(newCalendarId));
values.put (Events.DTSTART, dtStart);
values.put (Events.DTEND, dtEnd);
values.put (Events.EVENT_TIMEZONE, TimeZone.getDefault().getID());
values.put (Events.TITLE, title);
Uri uri = cr.insert (Events.CONTENT_URI, values);
// The returned uri will contain the eventId assigned by Events.