TestNetworkInterface: improve documentation of newly added methods

This change addresses API review feedback and improves documentation.
Specifically, it adds documentation for the newly added getMtu() and
getMacAddress() methods, describes under which conditions the MAC
address is null and adds a short blurb on how (or in what context) this
class is used.

Test: TH
Bug: 236187143
Change-Id: I349897fd55f3085ce88c8b94c84ec636bb5eb2a6
1 file changed