Implement FileSystemRegistry.listRoots.

This patch implements 
`public static java.util.Enumeration listRoots()`.

Currently we only have one root with the url `file:///`. 
FileSystemRegistry is part of JSR75. According to the spec as following, the root name should be "". So `listRoots()` returns an Emuneration of a single element "".

JSR75 spec for FileSystemRegistry.listRoots
----

This method returns the currently mounted root file systems on a device as String objects in an Enumeration. If there are no roots available on the device, a zero length Enumeration is returned. If file systems are not supported on a device, a zero length Enumeration is also returned (this check is performed prior to security checks).

The first directory in the file URI is referred to as the root, which corresponds to a logical mount point for a particular storage unit or memory. Root strings are defined by the platform or implementation and can be a string of zero or more characters ("" can be a valid root string on some systems) followed by a trailing "/" to denote that the root is a directory. Each root string is guaranteed to uniquely refer to a root. Root names are device specific and are not required to adhere to any standard. Examples of possible root strings and how to open them include:

    Possible Root Value | Opening the Root
    ------------------------- | --------------------------
    CFCard/  | Connector.open("file:///CFCard/");
    SDCard/  | Connector.open("file:///SDCard/");
    MemoryStick/ | Connector.open("file:///MemoryStick/");
    C:/  | Connector.open("file:///C:/");
    / 	Connector.open("file:////");

    The following is a sample showing the use of listRoots to retrieve the available size of all roots on a device:

       Enumeration rootEnum = FileSystemRegistry.listRoots();
       while (e.hasMoreElements()) {
          String root = (String) e.nextElement();
          FileConnection fc = Connector.open("file:///" + root);
          System.out.println(fc.availableSize());
       } 
     

    Returns:
        an Eumeration of mounted file systems as String objects. 
    Throws:
        java.lang.SecurityException - if application is not given permission to read files.
    See Also:
        FileConnection