Add BPF support for NetworkStatsService

Add the native implementation for NetworkStatsService to read the
uidStatsMap and tagStatsMap to get the network traffic information.
Currently the implementation only try to get the data from bpf for a
reference but the final value returned is still from old xt_qtaguid
module. So the result will not be effected. The bpf implementation is
depending on libbpf inside netd.

Bug: 30950746
Test: CtsNetTestCases -t android.net.cts.TrafficStatsTest

Change-Id: Ia85e4d4d602f6bd536739d89085e6dc37ad82e15