When a calling the updateInterface API, if no eth specifier was set it
would be removed not allowing follow-up network requests to request by
ethernet specifier.
This CL makes sure that the eth specifier is always set as expected when
updateInterface is called by either validating the input or setting the
specifier if the passed one is null.
Bug: 236294399
Bug: 229207021
Test: atest FrameworksNetTests
:com.android.server.ethernet.EthernetNetworkFactoryTest
Change-Id: Ic05ca7fe4adaa94f79e59758569f7595ab4c4f54
59d98c07a7