ZipConstants64 defines duplicate constants EXTID_ZIP64 and ZIP64_EXTID

ZipConstants64 two different constants for the Zip64 Extended Information Extra Field Header ID (see APPNOTE.TXT 4.5.3).

```
    static final int ZIP64_EXTID = 0x0001; // Extra field Zip64 header ID
...
    static final int EXTID_ZIP64 = 0x0001; // Zip64
```

ZIP64_EXTID and EXTID_ZIP64 are both used in the implementation. ZIP64_EXTID was added first, EXTID_ZIP64 was added in JDK-8024338 (which moved a constant originally added to ZipConstants to ZipConstants64).

It think it's worth considering consolidating on one of the constants, to make it easier to review all of the places that handle Zip64 Extended Information extra fields.