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/compiler/dex/dex_to_dex_decompiler_test.cc b/compiler/dex/dex_to_dex_decompiler_test.cc
index 7eaba96..88426a3 100644
--- a/compiler/dex/dex_to_dex_decompiler_test.cc
+++ b/compiler/dex/dex_to_dex_decompiler_test.cc
@@ -82,13 +82,7 @@
         continue;
       }
       ClassDataItemIterator it(*updated_dex_file, class_data);
-      // Skip fields
-      while (it.HasNextStaticField()) {
-        it.Next();
-      }
-      while (it.HasNextInstanceField()) {
-        it.Next();
-      }
+      it.SkipAllFields();
 
       // Unquicken each method.
       while (it.HasNextDirectMethod()) {