Change intern table to unordered set.

Intern table active used bytes goes from 430k to 317k on system
server. Similar %wise savings on other apps.

Bug: 16238192

(cherry picked from commit d910fcef539e12ab181e56ec80684f39c4e95733)

Change-Id: Ic70395124435c6f420a77e6d8639404a160f395a
diff --git a/runtime/mirror/string.h b/runtime/mirror/string.h
index 66a5dd8..1320ab7 100644
--- a/runtime/mirror/string.h
+++ b/runtime/mirror/string.h
@@ -66,7 +66,8 @@
 
   int32_t GetHashCode() SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
 
-  void ComputeHashCode() SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
+  // Computes, stores, and returns the hash code.
+  int32_t ComputeHashCode() SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);
 
   int32_t GetUtfLength() SHARED_LOCKS_REQUIRED(Locks::mutator_lock_);