Revert "Revert "ART: Give JIT thread pool workers a peer""

This reverts commit 9dfb707b.

Accept a live Java thread for the JIT, and adjust the tests
accordingly.

Bug: 31684920
Test: m ART_TEST_JIT=true ART_TEST_INTERPRETER=true test-art-host
Change-Id: I92cbae1eaae05711b9069335cf1a5f7eb58b9fd8