Reset NativeLoader on DestroyJavaVM

Native loader keeps vector of weak references
to classloader. ResetNativeLoader instructs
it to clear this vector once JavaVM is
destroyed.

Bug: http://b/28453840
Change-Id: I45e38c45268a741d86ec686266b13fde7d5fb9a2
(cherry picked from commit 9e83422b27faa3f249ec32ad2cbaa34530bd3df1)
diff --git a/runtime/java_vm_ext.cc b/runtime/java_vm_ext.cc
index 35bb3c3..79c3203 100644
--- a/runtime/java_vm_ext.cc
+++ b/runtime/java_vm_ext.cc
@@ -318,6 +318,7 @@
     }
     JavaVMExt* raw_vm = reinterpret_cast<JavaVMExt*>(vm);
     delete raw_vm->GetRuntime();
+    android::ResetNativeLoader();
     return JNI_OK;
   }