Add atomic method reference map

Less RAM usage and faster than using a normal map with
MethodReference. Speed is faster by avoiding locking and tree
traversal. RAM usage is lower since the map usually had a value
for most method references.

Plan on using for marking methods for dex2dex, storing compiled
methods. Also use the new map for VerifiedMethods (refactoring).

Added test.

Bug: 32641252

Test: test-art-host-run-test

Change-Id: I46268031b8e0daf9be3597145cf6ecf579a039e2