ART: Change shutdown order

Add explicit Shutdown method to ThreadList, doing the work that
was previously in the destructor. Call sid method before unloading
plugins and stopping the debugger.

Modulo daemon threads, this ensures that thread events will have
quiesced when plugins stop.

Bug: 37763974
Test: m test-art-host
Change-Id: I11d22f2c89a68e9f6720d61967b8e5aea41188dc