cutils: add a way to disable tracing for a process

This change adds the atrace_set_tracing_enabled call to libcutils.  The call
can be used to disable all tracing (of the atrace variety) for the current
process.  This is to be used to disable tracing in the Zygote process, as there
is no way for Zygote to be notified of changes to the enabled trace tags.

Change-Id: I0b691cc0dcfc65b16e3d17e1db2866a4deb253a7