Add shims for ConnectivityManager

requestBackgroundNetwork and registerSystemDefaultNetworkCallback
are not available before API31 so ConnectivityManageShim is used
as a wrapper that can hold a reference to it in common code.

Bug: 167645754
Test: m NetworkStackApiCurrentLib NetworkStackApiStableLib
Test: atest NetworkStackTests NetworkStackNextTests
Test: local verify the usage in ConnectivityManagerTests
Change-Id: I1f7ecd6c6005631951f514d60885a5d453c3f2d9