traced_probes: Put all statsd atoms into a single batch

The main change is that rather than ending up with a packet per atom
we now end up with many atoms in a single packet.

There are a couple of small tweaks to achieve this:
- Now the root timestamp on the packet is always the time we
  are *receiving* the atom batch in boot time. This aids sorting
  in trace_processor until we sort out some special sorting for atoms.
- Remove [packed=true] from ShellData timestamp_nanos. statsd does not
  actually pack this field and protozero is quite picky about
  packed field which makes it hard to parse. We don't actually need to
  parse it after this change but I didn't want to leave it around to
  trip up the next person.
- Rather than parsing each ShellData one atom at a time and inserting
  them into the trace individually now we take all whole contents of
  ShellData and put it in a StatsdAtom track packet (whose format
  matches exactly precisely so this was possible.)

$ adb push test/configs/statsd.cfg /data/misc/perfetto-configs/statsd.cfg
$ adb shell perfetto --txt -c /data/misc/perfetto-configs/statsd.cfg -o /data/misc/perfetto-traces/statsd.pb
$ adb pull /data/misc/perfetto-traces/statsd.pb statsd.pb
$ ninja -C out/r traceconv && out/r/traceconv text statsd.pb

Test: See above.
Change-Id: I48ebb10c56ffbfbca14eb4db3962218a740fbb19