I cannot find Standard Assets (Mobile) when trying to import it in Unity 5, nor can I find the files online.
Unity Updated and restructured the Standard Assets in Unity5 and the old one are no longer available.
If you need a mobile ready input you can take a look at the “CrossPlatformInput” package.
For anyone still looking the cross platform controls are in the Standard Assets Bundle… Quite big - you might want to just select what u want…