Bug-116 Make MainActivity can select different category.

There is a logic error at line 22. We may have category = CATEGORY_NAMES_ID.length to pass and get an IndexOutOfBoundException.

Please rewrite as the following:
```
return (category > -1 && category < CATEGORY_NAMES_ID.length) ? 
           cxt.getString(CATEGORY_NAMES_ID[category - 1]) : null;
```

Use `&&` will give better performance. Because once the first expression is false, it doesn't need to evaluate the second expression.