Defer registration of the arc4random fork-detect handler.

Previously, arc4random would register a fork-detecting pthread_atfork
handler to not have to call getpid() after a fork. pthread_atfork uses
pthread_mutex_lock, which requires the current thread to be initialized,
preventing the use of arc4random for initializing the global stack guard,
which needs to happen before the main thread has been initialized.

Extract the arc4random fork-detection flag and use the existing
arc4random fork handler to set it.

Bug: http://b/29622562
Change-Id: I98c9329fa0e489c3f78cad52747eaaf2f5226b80