Move CompatabilityCloseMonitor to AsynchronousCloseMonitor's C API.

This is orthogonal to any future plans for AsynchronousCloseMonitor
and required because the C++ API is no longer public.

Bug: 123121395
Test: atest CtsLibcoreOkHttpTestCases:com.squareup.okhttp.CallTest
Change-Id: I994a50e2bacd26b79b1a84680bc76eb2e50afb78