perfetto: allow traced to write into FDs received by the client

This allows an optimization that consists in the "perfetto" cmdline
client passing directly the file descriptor for the output trace
to traced (as opposite to having traced streaming back the trace
data to "perfetto" and having that one doing the write() into file).
This reduces sensibly the memory traffic and CPU overhead of traces
with a minor change.

Bug: 73625179
Test: builds + perfetto_integrationtests w/ long_trace.cfg
Change-Id: I81f5a230338ced20dc543fd91c5a0bd0e58725f2