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/imgdiag/imgdiag_test.cc b/imgdiag/imgdiag_test.cc
index 82bc8b9..0d6a8c9 100644
--- a/imgdiag/imgdiag_test.cc
+++ b/imgdiag/imgdiag_test.cc
@@ -42,7 +42,7 @@
     CommonRuntimeTest::SetUp();
 
     // We loaded the runtime with an explicit image. Therefore the image space must exist.
-    gc::space::ImageSpace* image_space = Runtime::Current()->GetHeap()->GetImageSpace();
+    gc::space::ImageSpace* image_space = Runtime::Current()->GetHeap()->GetBootImageSpace();
     ASSERT_TRUE(image_space != nullptr);
     boot_image_location_ = image_space->GetImageLocation();
   }