Do not disable tracing in zygote.

We now use the sequence number of the atrace tag rather than a Binder
transaction, so zygote can observe changes to the tags.

Test: m
Bug: 137366208
Bug: 147667830

This is a cherry-pick of e38797be.

Change-Id: I50e32a19c538dd255097c035836504b4746b172b
Merged-In: I50e32a19c538dd255097c035836504b4746b172b