native: Cleanup jni usage code

Make the C++ signature consistent with the JNI descriptor by using the
same type as the descriptor whenever possible.

e.g. "()Ljava/lang/String;" should actually be "jstring fn(JNIEnv*,jobject)"
instead of "jobject fn(JNIEnv*,jobject)".

Bug: 35325126
Change-Id: I72318525fc3b18b013b8d6fa604d8dd6b5dd4400
Test: make -j32 test-art-host  # and also manually that it boots
diff --git a/runtime/native/java_lang_reflect_Field.cc b/runtime/native/java_lang_reflect_Field.cc
index f990c04..688ae19 100644
--- a/runtime/native/java_lang_reflect_Field.cc
+++ b/runtime/native/java_lang_reflect_Field.cc
@@ -460,10 +460,10 @@
   return reinterpret_cast<jlong>(field);
 }
 
-static jobject Field_getNameInternal(JNIEnv* env, jobject javaField) {
+static jstring Field_getNameInternal(JNIEnv* env, jobject javaField) {
   ScopedFastNativeObjectAccess soa(env);
   ArtField* field = soa.Decode<mirror::Field>(javaField)->GetArtField();
-  return soa.AddLocalReference<jobject>(
+  return soa.AddLocalReference<jstring>(
       field->GetStringName(soa.Self(), true /* resolve */));
 }