ART: Fix destruction order in Runtime.

Destroy arena allocators as late as possible.

With https://android-review.googlesource.com/172057 , the
ArenaPool was indirectly used by ClassLinker's destructor
(via ~LinearAlloc) after it has already been destroyed.
These allocators were being destroyed way too early anyway,
they should be orderly destroyed when the unique_ptr<>
members of Runtime have their destructors executed. However,
due to the potential dependence on MemMap, we destroy them
just before the MemMap::ShutDown().

Change-Id: Ifc3e5a24a29536d5767c82353901e2d22b13d643