sepolicy: Allow system server to r/w the bpf maps

The system server is responsible for providing the network traffic
stats to Apps and services. Allow it to directly reading the eBPF maps
that stored these information can make the process of getting traffic
stats simplier.

Test: No selinux rule violation of system server reading netd bpf object
Bug: 30950746
Change-Id: I6d9438d1ed7c9bab45a708f5d2a85eb22f5e8170