ART: Add ThreadPool mode that creates peers

Add a mode where worker threads in a thread pool will get a Java
peer. Add a test.

(cherry picked from commit b15de0c0)

Bug: 29547798
Bug: 31684920
Test: m test-art-host-gtest-thread_pool_test
Change-Id: I3654cc2be1294a79881b6ac9f84445a1e7f24b70