Add tracing for network statistics and policy.

These trace points surround areas of heavy lifting to help us debug
code that might be taking longer than expected.  (For example, slow
getting data from eBPF, slow recording into data structures, slow
writing to disk, or slow evaluating policy.)

Typical timings on a taimen:

performPollLocked:   25.5ms
snapshotUid:          4.4ms
snapshotXt:           1.4ms
[persisting]:        13.4ms
updateNotifications:  3.3ms

Bug: 77908520, 77808546, 77853238, 77154412
Test: builds, boots
Change-Id: I072bfecffee01eeec7e4cbad439bd1cdc166febc