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/runtime.cc b/runtime/runtime.cc
index b868563..ca65c2b 100644
--- a/runtime/runtime.cc
+++ b/runtime/runtime.cc
@@ -559,7 +559,10 @@
       "getSystemClassLoader", "()Ljava/lang/ClassLoader;", pointer_size);
   CHECK(getSystemClassLoader != nullptr);
 
-  JValue result = InvokeWithJValues(soa, nullptr, soa.EncodeMethod(getSystemClassLoader), nullptr);
+  JValue result = InvokeWithJValues(soa,
+                                    nullptr,
+                                    jni::EncodeArtMethod(getSystemClassLoader),
+                                    nullptr);
   JNIEnv* env = soa.Self()->GetJniEnv();
   ScopedLocalRef<jobject> system_class_loader(env, soa.AddLocalReference<jobject>(result.GetL()));
   CHECK(system_class_loader.get() != nullptr);