[Metadata] Added getArtwork method of MetadataFileHandle

[Metadata] Added getArtwork method of MetadataFileHandle

[ACR] https://code.sec.samsung.net/jira/browse/TWDAPI-271

[Verification]
Code compiles without errors.
Checked in chrome console with below code.
Precondition: sample.mp3 with artwork need to be pushed on device

// test artwork
var path = "videos/sample.mp3";
function showBlob(bbb) {
    var elem = document.getElementById("blobImage") || document.createElement("img");
    elem.setAttribute("id", "blobImage");
    elem.setAttribute("height", "240");
    elem.setAttribute("width", "240");
    elem.setAttribute("alt", "invalid blob");
    document.getElementById("test").appendChild(elem)

    var objectURL = URL.createObjectURL(bbb);
    elem.src = objectURL;
}
var fileHandle = tizen.metadata.createFileHandle(path);
var artwork = fileHandle.getArtwork();
console.log(artwork);
showBlob(artwork)

Change-Id: Ia0cbe5ade542ad71224a91c0c1a4618072b5538e