Fix a race condition on GC timing logger data.

The GC timing logger data is thread local to the GC thread and
shouldn't be used by a mutator. The original intention was to have a
systrace scope.

This should fix the 004-JniTest failures.

Bug: 30980189
Bug: 29517059
Bug: 12687968
Test: test-art-host
Change-Id: Ibc8cfbfdd64d85ff65bf220d3022fc8fdf1064f2