atrace: don't write the clock sync marker if --only_userspace

This CL has no other intended side effect than reducing
spam in logs.
--only_userspace is effectively used only by Perfetto.
Today the unconditional invocation of writeClockSyncMarker()
writes consistently, because perfetto invokes atrace before
writing 1 to tracing_on, and that makes the write to
trace_marker fail.
The failure has always been there and has always been fine.
This CL just silences it. I am planning to bubble up non-fatal
errors coming from atrace to the UI, this would be a confusing
false positive.

Bug: 171085599
Test: manual: perfetto -t 1s bionic am foobar -o trace;
              check logcat (no clock sync errors)
Change-Id: Ic9b9cef42465bae0214e73c0dbce9ed2d288fdc6