Enable Ethernet if the device has FEATURE_USB_HOST.

Any device that supports USB OTG is capable of running Ethernet
via a USB OTG cable and Ethernet to USB adapter.

Currently, we only start the Ethernet system service if the
device has FEATURE_ETHERNET, but this requires that every device
explicitly declare FEATURE_ETHERNET, which causes bugs like
http://b/18515146 , where the L OTA broke Ethernet on nakasi.

Therefore, start the Ethernet service on all devices that have
FEATURE_USB_HOST.

Bug: 18515146
Change-Id: I3b4e85d1ad8e1aea9baa046a27f5b4dd68c42028