Fix Ethernet documentation.

1. Remove incorrect @return tag for void function.
2. Fix build breakage by removing the reference to
   #ETHERNET_SERVICE, which is hidden.

Change-Id: Ia893464bafc8257e48e0bd710d27954b535fcece

core/java/android/net/EthernetManager.java

@@ -47,7 +47,7 @@ public class EthernetManager {
     }
 
     /**
-     * Get Ethernet configuration
+     * Get Ethernet configuration.
      * @return the Ethernet Configuration, contained in {@link IpConfiguration}.
      */
     public IpConfiguration getConfiguration() {
@@ -61,8 +61,7 @@ public class EthernetManager {
     }
 
     /**
-     * Set Ethernet configuration
-     * @return true if setting success
+     * Set Ethernet configuration.
      */
     public void setConfiguration(IpConfiguration config) {
         try {