Add LinkProperties methods to system APIs

The exposed methods are used by telephony, wifi
and tethering. For mainline support, making the
methods @SystemApi.

Bug: 139268426
Bug: 135998869
Bug: 138306002
Test: atest FrameworksNetTests
      atest NetworkStackTests
      atest FrameworksTelephonyTests
      ./frameworks/opt/net/wifi/tests/wifitests/runtests.sh
      atest android.net.cts
      atest android.net.wifi.cts
      atest android.telephony.cts

Change-Id: Ib16a838cf9f748e1c5b045d6c2f17678f16af28c