Add NATT keepalive resources and methods into IpSecService

This change adds a new NATTKeepalive resource type, along with the
associated allocation/deallocation. Additionally, this change allows
ReferenceCountedResource(s) to not be binder-linked, to allow the
ConnectivityService to verify ownership and allocate a NattKeepalive
without double-registering for binder-death notifications.

Bug: 125517194
Test: IpSecService frameworks tests ran
Change-Id: I8293f79940ad57dabb6f2b9de5e334d06b869443