Ensure shutdown thread has a java peer.

Using JVMTI it is possible to be running user-supplied code both on
the shutdown thread and on other threads while the shutdown thread is
running. Specifically some real-world JVMTI agents rely on being able
to suspend the shutdown thread while they do cleanup work. In order to
support this we will make sure that the shutdown thread is given its
own java peer so it can be suspended by other threads.

Bug: 66904725
Bug: 62821960

Test: ./test.py --host -j50

Change-Id: I1c89537c47fd3dd41aa4c0e7f7a6940d1c5ba913