Previously, the caller can only know about the transport type of
the underlying network. The information might not be enough if
the device support WiFi STA+STA.
Thus, provide an API for the caller to get the correct underlying
network.
Bug: 191918368
Test: atest FrameworksNetTests:NetworkCapabilitiesTest
Change-Id: I7752b2356770f4572f6ca4cbaecaa45c09d6d72f