Image space cleanup for app images

Removed Heap::GetImageSpace, added Heap::GetBootImageSpace.

Generalized some logic in the class linker for image spaces.

Bug: 22858531

Change-Id: Ib3e12bb061a247e232d3dc93b0d6b35eb3a34d25
diff --git a/compiler/image_test.cc b/compiler/image_test.cc
index a38e1f5..6df1527 100644
--- a/compiler/image_test.cc
+++ b/compiler/image_test.cc
@@ -181,7 +181,7 @@
   ASSERT_TRUE(heap->HasImageSpace());
   ASSERT_TRUE(heap->GetNonMovingSpace()->IsMallocSpace());
 
-  gc::space::ImageSpace* image_space = heap->GetImageSpace();
+  gc::space::ImageSpace* image_space = heap->GetBootImageSpace();
   ASSERT_TRUE(image_space != nullptr);
   ASSERT_LE(image_space->Size(), image_file_size);