[Android] Implement the function for file picker.

This patch is to implement the callback about file picker with
customized java code.
Move "showFileChooser" to XWalkWebChromeClient, move the related test
case to internal.

BUG=XWALK-2631, XWALK-2717
@wuhengzhi , you said it's moving from `XWalkContentsClient`? We definitely won't remove an API in XWalkUIClient which is public already.