Revert "Revert "Add one LinearAlloc per ClassLoader""

Issue was fixed by:
https://android-review.googlesource.com/#/c/171945/

Bug: 22720414

This reverts commit 7de5dfe37f3cf24e1166412b589f6f67dcd1f1c0.
diff --git a/runtime/gc/collector/concurrent_copying.cc b/runtime/gc/collector/concurrent_copying.cc
index 399591b..468179c 100644
--- a/runtime/gc/collector/concurrent_copying.cc
+++ b/runtime/gc/collector/concurrent_copying.cc
@@ -457,6 +457,8 @@
     CheckEmptyMarkStack();
     // Re-enable weak ref accesses.
     ReenableWeakRefAccess(self);
+    // Free data for class loaders that we unloaded.
+    Runtime::Current()->GetClassLinker()->CleanupClassLoaders();
     // Marking is done. Disable marking.
     DisableMarking();
     CheckEmptyMarkStack();