ART: Use thread_local on the host for ART Thread*

Use thread_local instead of a pthread key for Thread::Current().
Retain the pthread key as we are still using it for some sanity
checks in ThreadExitCallback.

Bug: 138329277
Test: m test-art-host
Change-Id: Idfe43bc279459e307e2165d3a3762af09230bdfd
4 files changed