Move Assemblers to the Arena.

And clean up some APIs to return std::unique_ptr<> instead
of raw pointers that don't communicate ownership.

Change-Id: I3017302307a0253d661240750298802fb0d9585e
diff --git a/runtime/base/arena_allocator.h b/runtime/base/arena_allocator.h
index 697f7e0..52a1002 100644
--- a/runtime/base/arena_allocator.h
+++ b/runtime/base/arena_allocator.h
@@ -88,9 +88,11 @@
   kArenaAllocRegisterAllocatorValidate,
   kArenaAllocStackMapStream,
   kArenaAllocCodeGenerator,
+  kArenaAllocAssembler,
   kArenaAllocParallelMoveResolver,
   kArenaAllocGraphChecker,
   kArenaAllocVerifier,
+  kArenaAllocCallingConvention,
   kNumArenaAllocKinds
 };