Avoid redundant allocations in WindowsPreferences

Found 2 redundant allocations in java.util.prefs.WindowsPreferences
1.
        if ((windowsName.length() > 1) &&
                (windowsName.substring(0, 2).equals("/!"))) {
            return toJavaAlt64Name(windowsName);
        }
No need to call substring here. We can use startWith instead.

2. String array allocated and then unused in a method childrenNamesSpi

    String[] subkeys = new String[subKeysNumber];