Using the Android Asset Studio
android:icon attribute, the value
@android:drawable/ic_menu_add references a system icon.
A system icon is one that is found on the device rather than in your project’s resources.
In a prototype, referencing a system icon works fine. However, in an app that will be released, it is better to be sure of what your user will see instead of leaving it up to each device. System icons can change drastically across devices and OS versions, and some devices might have system icons that do not fit with the rest of your app’s design.
One alternative is to create your own icons from scratch. You will need to prepare versions for each screen density and possibly for other device configurations. For more ...