Avoid std::string allocations for finding an array class.

Introduce ClassLinker::FindArrayClass which performs an array class lookup
given the element/component class. This has a 16 element cache of recently
looked up arrays.
Pass the current thread to ClassLinker Find .. Class routines to avoid calls
to Thread::Current().
Avoid some uses of FindClass in the debugger where WellKnownClasses is a
faster and more compacting GC friendly alternative.

Change-Id: I60e231820b349543a7edb3ceb9cf1ce92db3c843
diff --git a/compiler/image_test.cc b/compiler/image_test.cc
index 3406fe6..49cabdc 100644
--- a/compiler/image_test.cc
+++ b/compiler/image_test.cc
@@ -151,7 +151,7 @@
   for (size_t i = 0; i < dex->NumClassDefs(); ++i) {
     const DexFile::ClassDef& class_def = dex->GetClassDef(i);
     const char* descriptor = dex->GetClassDescriptor(class_def);
-    mirror::Class* klass = class_linker_->FindSystemClass(descriptor);
+    mirror::Class* klass = class_linker_->FindSystemClass(soa.Self(), descriptor);
     EXPECT_TRUE(klass != nullptr) << descriptor;
     if (image_classes.find(descriptor) != image_classes.end()) {
       // Image classes should be located inside the image.