Updating long_trace.cfg
Updating long_trace.cfg

Adding flush_period_ms for long trace config.

Otherwise, traceconv will display the following warning message:

  It is strongly recommended to have flush_period_ms set when
  write_into_file is turned on. You will likely have many dropped events
  because of inability to sort the events correctly.

Bug: None
Test: adb shell perfetto --txt \
        -c /data/misc/perfetto-traces/long_trace.cfg \
        -o /data/misc/perfetto-traces/trace_log
Test: ./traceconv systrace trace_log trace_log.systrace, checks there is
      no warning message
Change-Id: I0f594ae3fb243f916db823114ee229b7a15965ee

google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto