Add field/method skipping helper functions to DexFile

Specifically added SkipStaticFields, SkipInstanceFields,
SkipAllFields, SkipStaticMethods, and SkipVirtualMethods.

Also applied the helpers to the code base.

Test: test-art-host
Change-Id: Idda77f9a6e2564c2e588d2bbe11cc320063fdb32
diff --git a/runtime/art_method.cc b/runtime/art_method.cc
index d591e09..32946ef 100644
--- a/runtime/art_method.cc
+++ b/runtime/art_method.cc
@@ -433,13 +433,7 @@
   const uint8_t* class_data = dex_file.GetClassData(class_def);
   CHECK(class_data != nullptr);
   ClassDataItemIterator it(dex_file, class_data);
-  // Skip fields
-  while (it.HasNextStaticField()) {
-    it.Next();
-  }
-  while (it.HasNextInstanceField()) {
-    it.Next();
-  }
+  it.SkipAllFields();
   // Process methods
   size_t class_def_method_index = 0;
   while (it.HasNextDirectMethod()) {