libui: Add hasSignaled() method to Fence

    // occurs then -1 is returned.

    nsecs_t getSignalTime() const;

    // hasSignaled returns whether the fence has signaled yet. Prefer this to

    // getSignalTime() or wait() if all you care about is whether the fence has

    // signaled.

    inline bool hasSignaled() {

        // The sync_wait call underlying wait() has been measured to be

        // significantly faster than the sync_fence_info call underlying

        // getSignalTime(), which might otherwise appear to be the more obvious

        // way to check whether a fence has signaled.

        return wait(0) == NO_ERROR;

    }

    // Flattenable interface

    size_t getFlattenedSize() const;

    size_t getFdCount() const;