ART: Change contention logging lock strategy

Compute possible contention (wrt/ sampling) first. Then keep the whole
logging section under the mutator lock *and* the thread-list lock so
as to avoid any thread races.

This change is in preparation for heavier logging under heavy lock
contention.

Enable contention logging in 004-ThreadStress.

Bug: 62353392
Test: m test-art-host
Change-Id: I7b9126bae273f0cddb681f5f013ce6d814c7b8c0