Tag network sockets.

This patch tags all update_engine network sockets with 0x55417243
("CrAU" in little-endian) so its network data usage can be tracked.

Bug: 35721166
Test: Ran uniitetst; /proc/net/xt_qtaguid/stats shows this tag.
Change-Id: I8a4076f2958d493a59a5e73af0b6b54f1733e7f2