Replace InterfaceParams with ifName in DhcpServer

The current code only uses InterfaceParams#name, and InterfaceParams is
defined in services/net which DhcpServer cannot depend on once moved to
a separate app.

Test: atest FrameworksNetTests
Bug: b/112869080
Change-Id: I94c7dce33200c111666a9dddde82ac2e66a6794f