Better network stats parsing, integer tags, async.

Change NMS parsing to handle extended /proc/ stats formats by pairing
values with header keys.  Move TrafficStats to integer tags to match
kernel internals, and offer well-known tags for system services.

Async policy event dispatch from NPMS, and update tests to block for
event dispatch.  Narrow app policy to exclude apps signed with system
key, which are usually critical.

Bug: 4948913, 4903489, 4585280

Change-Id: Idb357227ccaa617906411f309371cea18d7bc519