Disable tracing from Zygote

This change disables all atrace tracing in Zygote immediately after it is
initialized.  This is necessary because Zygote has no way to receive
notifications that the enabled trace tags have been changed.  Tracing is
re-enabled when other processes fork from Zygote.

Change-Id: If2983858fb0c4890ba9ab041849b1c4d98f66c13