Make states atomic and update comments

void Session::enterStateOrCrash(int cookie, SessionState state) {

    CHECK(mScheduledState == state);

    mCurrentState = mScheduledState;

    mCurrentState = state;

    mScheduledState = SessionState::IDLING;

    mCb->onStateChanged(cookie, mCurrentState);

}

    // by calling ISessionCallback#onStateChanged.

    void enterIdling(int cookie);

    // The sensor and user IDs for which this session was created.

    int32_t mSensorId;

    int32_t mUserId;

    // Callback for talking to the framework. This callback must only be called from non-binder

    // threads to prevent nested binder calls and consequently a binder thread exhaustion.

    // Practically, it means that this callback should always be called from the worker thread.

    std::shared_ptr<ISessionCallback> mCb;

    // Module that communicates to the actual fingerprint hardware, keystore, TEE, etc. In real

    // life such modules typically consume a lot of memory and are slow to initialize. This is here

    // to showcase how such a module can be used within a Session without incurring the high

    // initialization costs every time a Session is constructed.

    FakeFingerprintEngine* mEngine;

    // Worker thread that allows to schedule tasks for asynchronous execution.

    WorkerThread* mWorker;

    SessionState mScheduledState;

    SessionState mCurrentState;

    // Simple representation of the session's state machine. These are atomic because they can be

    // modified from both the main and the worker threads.

    std::atomic<SessionState> mScheduledState;

    std::atomic<SessionState> mCurrentState;

};

}  // namespace aidl::android::hardware::biometrics::fingerprint