Add OsCompat

Although the InetAddress symbols used by Connectivity are stable core
platform API, and should be usable, the core_current stubs are not yet
part of the module_current API.
Until that is fixed, add an InetAddressCompat utility that calls the
three static methods by reflection.

Test: atest FrameworksNetTests CtsNetTestCases
Bug: 183097033
Change-Id: I8d1a4811a1cc7dbc62f4033c2b320246d1d72b28