Don't initialize atrace if it's already disabled.

For processes that start very early, atrace initialization can cause
an selinux denial, so make sure it's possible to disable it and avoid
the call to open "trace_marker".

Bug: 63927601
Test: disable atrace early in vold, ensure that selinux denial is
avoided.

Change-Id: I2422e6d0db323bc13c6d6ed1896435151fca21f7