Build EthernetService.

This is included in the PRODUCT_BOOT_JARS because it needs to be
loaded by system_server via SystemServiceManager.startService,
which only looks at the boot classpath.

This is compiled on all devices as opposed to only devices with
a physical Ethernet port because we also support Ethernet over
USB.

Change-Id: I55bc422e2a7b700eea4b7513f96079502be36882