Instead of having statsd linking the perfetto client library
and talk directly to its socket, we let just statsd exec()
the /system/bin/perfetto cmdline client.

There are two reasons for this:
1) Simplify the interaction between statsd and perfetto, reduce
  dependencies, binary size bloat and isolate faults.
2) The cmdline client also takes care of handing the trace to
  Dropbox. This allows to expose the binder interaction surface
  to the short-lived cmdline client and avoid to grant binder
  access to the perfetto traced daemon.

This cmdline client will be used by:
 - statsd
 - the shell user (for our UI and Studio)

Bug: 70942310
Change-Id: I8cdde181481ad0a1a5cae5937ac446cedac54a1f