Fix sepolicy for bpf object

With the new patches backported to 4.9 kernels, the bpf file system now
take the same file open flag as bpf_obj_get. So system server now need
read permission only for both bpf map and fs_bpf since we do not need
system server to edit the map. Also, the netd will always pass stdin
stdout fd to the process forked by it and do allow it will cause the
fork and execev fail. We just allow it pass the fd to bpfloader for now
until we have a better option.

Test: bpfloader start successful on devices with 4.9 kernel.
      run cts -m CtsNetTestCases -t android.net.cts.TrafficStatsTest
Bug: 74096311
Bug: 30950746

Change-Id: I747a51cb05ae495c155e7625a3021fc77f921e0d