ART: Refactor UnstartedRuntime for testing

Expose the UnstartedRuntime implementation functions as private static
methods of a class. Add a gtest that can invoke these functions. Add
sample tests for String and Memory.

Bug: 21173514
Change-Id: Ib5bde6347fafaf7607c642542ea7d5938ff4b1df
diff --git a/runtime/common_runtime_test.cc b/runtime/common_runtime_test.cc
index 7a711cc..de3a29b 100644
--- a/runtime/common_runtime_test.cc
+++ b/runtime/common_runtime_test.cc
@@ -327,7 +327,7 @@
   // Initialize maps for unstarted runtime. This needs to be here, as running clinits needs this
   // set up.
   if (!unstarted_initialized_) {
-    interpreter::UnstartedRuntimeInitialize();
+    interpreter::UnstartedRuntime::Initialize();
     unstarted_initialized_ = true;
   }