Add new APIs in NetworkCapabilities to set and get underlying networks

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
3 files changed