Initialize art::Atomic's value to 0.

art::Mutex and art::ConditionVariable were DCHECKing that the value was
zero on entering the constructor without ever initializing the value.
Since 0 is a sensible default, make it the default for art::Atomic
rather than initializing for each use.

Bug: 16301104
Change-Id: I9c98b82c80670b7a6c53d46a08236bbed6a64f8f