Support for hotspot client visibility.

Adding support for visibility into clients that connect to an active hotspot.

Bug: 137309578
Test: atest FrameworksNetTests:com.android.server.connectivity.TetheringTest
atest FrameworksWifiApiTests:android.net.wifi.WifiManagerTest
atest FrameworksWifiApiTests:android.net.wifi.WifiClientTest
Tested manually on Pixel 3.

Change-Id: I1caeb10bc50202873e760a76b346bccd941e2574
Merged-In: I1caeb10bc50202873e760a76b346bccd941e2574