1. Add libnetd_updatable.so in com.android.tethering. The library is
loaded by netd. Currently, it mainly targets on a few functions which
access BPF maps. The functionality may extend in the future.

2. Attach gcroup progs from libnetd_updatable.so.

3. Move (privileged)TagSocket and untagSocket implementation to mainline
module. Combine privilegedTagSocket and untagSocket into a single
function.

4. Split related unit tests from netd_unit_test to
libnetd_updatable_unit_test as well.

Bug: 202086915
Test: cd system/netd; atest
Test: atest TrafficStatsTest NetworkUsageStatsTest

Change-Id: Ib556458103a4cbb643c1342d9b689ac692160de0