Initialize art::Atomic's value to 0.

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

(cherry picked from commit 6a3f8d93ddf09b5f6667820089e488958cba8361)

Change-Id: Iab69bccac9f904111a72eff8162c7f0ca979a08a