check this:
https://github.com/android/platform_frameworks_base/blob/master/core/java/android/net/EthernetManager.java
http://stackoverflow.com/questions/21746192/ethernet-connectivity-through-programatically-android-rooted-device
and
if a rooted device (don't do this)
you can execute some linux shell cmd from your java code.this may do the trick.