(fc) Avoiding AtomicBoolean in FileInput/-OutputStream improves startup

JDK-8025619 changed the implementation from a volatile boolean + lock to AtomicBoolean, which was rather benign up until when VarHandles was integrated, which means initializing AtomicBoolean now pulls in 20 VarHandle classes and does a non-trivial amount of initialization.

Since we always create FileInput/-OutputStreams during system initialization (System.in/out/err), deferring initialization of AtomicBoolean appears worthwhile.