base::Daemonize(): wait for child setsid()

When invoking 'adb shell perfetto --background ...'
the parent (perfetto --background) prints the child pid and
returns immediately. If the child has not had a chance to
be scheduled and has not run past the setsid(), it will be
killed via a SIGHUP as well, which breaks the expectations of
'adb shell perfetto --background'.
Interestingly most daemon() in various libc() implementations
seem to be affected by the same bug.

Bug: 238644870
Change-Id: I21084a0f3af022255a605705ea4dc60e62cab555