Use AtomicDexRefMap for compiled classes

Changed compiled_classes_ to use an AtomicDexRefMap and deleted
the lock since it was no longer necessary.

This map is more compact than a SafeMap. RAM numbers for verify
filter on a large app (host arm compile):

Maximum resident set size (kbytes): 250012->243472
native alloc: 19284128B->13951600B

Bug: 6346774
Test: test-art-host
Change-Id: Iace66945b49433f353603a713593c53be6893cc5