ART: Refactor jmethodID handling

ArtMethod objects have been native since Marshmallow.
Remove the dependency on being runnable. Refactor the
code into the jni_internal header.

Test: m test-art-host
Change-Id: I1385fcd4c08981491701da55a87036b447aa2fc2
diff --git a/runtime/thread.cc b/runtime/thread.cc
index 3f7d086..1e3d4db 100644
--- a/runtime/thread.cc
+++ b/runtime/thread.cc
@@ -2414,7 +2414,7 @@
       ++i;
     }
     ScopedLocalRef<jobject> ref(soa.Env(), soa.AddLocalReference<jobject>(exception.Get()));
-    InvokeWithJValues(soa, ref.get(), soa.EncodeMethod(exception_init_method), jv_args);
+    InvokeWithJValues(soa, ref.get(), jni::EncodeArtMethod(exception_init_method), jv_args);
     if (LIKELY(!IsExceptionPending())) {
       SetException(exception.Get());
     }