Change a few places to use string version of GetTypeId

GetTypeId is about 20% faster than GetStringId + integer GetTypeID
since it does less binary searches.

Change-Id: I876c4ac89ab206acca217b2287b0197ef2e408c2
diff --git a/runtime/dex_file.cc b/runtime/dex_file.cc
index b3ca6ac..3a93aac 100644
--- a/runtime/dex_file.cc
+++ b/runtime/dex_file.cc
@@ -738,11 +738,7 @@
     }
     // TODO: avoid creating a std::string just to get a 0-terminated char array
     std::string descriptor(signature.data() + start_offset, offset - start_offset);
-    const DexFile::StringId* string_id = FindStringId(descriptor.c_str());
-    if (string_id == nullptr) {
-      return false;
-    }
-    const DexFile::TypeId* type_id = FindTypeId(GetIndexForStringId(*string_id));
+    const DexFile::TypeId* type_id = FindTypeId(descriptor.c_str());
     if (type_id == nullptr) {
       return false;
     }