Add apex_available to libnetd_client

Add apex_available to libnetd_client, so it can be used by
framework JNI code that is in the process of moving to the
connectivity (tethering) APEX.

Bug: 171540887
Test: m (APEX builds with additional dependencies added)
Change-Id: Iebfa4f741e81357e2d6e4ec197294575999f9a6c