Fix major WifiScanner bugs

@Djfe good catch!
This fixes #1216, the feature was removed in #853
If I do this, Android Studio does not recognize the SDK version check and warns about using the function, which isn't really nice. It seems to recognize it only if the version check is the only condition in the `if` statement.
Since this is not a hot function, I left it like this. Having two nested `if`s would also work, but does not look nice. Or just not care about Android Studio and use your suggestion anyway?
![as](https://cloud.githubusercontent.com/assets/3845150/4938865/b5a6468e-65ce-11e4-8a35-556343740598.png)