Attempt to fix NetworkTracePollerTest flake

The test has flaked twice, once reporting only half of the packets, the
second time reporting no packets. Although I haven't been able to repro
this locally with 500 attempts, I believe it's a race between the kernel
thread writing and test thread receiving the events. This adds a retry
with a brief sleep only if we don't get all packets on the first try.

This also records all unmatched packets in case something like the port
or uid reporting breaks. Rather than fail saying zero packets, you'll
see the ones we skipped and can (hopefully) tell what went wrong.

Bug: 273600719
Test: atest libnetworkstats_test
Change-Id: Iee21f30a8dc59be5649f8e8b6509f4cc69ae5ff9