Try to fix and diagnose Perfetto flush errors.

Around 10% of the traces with Nettrace have "traced_final_flush_failed"
errors. It is believed that Network Tracing isn't writing enough data to
fill one "Chunk" in Perfetto's buffer. Although this should still be
saved by Perfetto, it doesn't seem to be.

This change records the number of packets read from the ring buffer to
understand whether the error coincided with low-data cases. It also
tries to flush the data OnStop to potentially fix the issue.

Test: flash and run trace
Change-Id: I92c8d2d8d47d1ed123585e1cfdde802d286f120f