[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
The distinction between the Internal and non-Internal java files is still unclear to me, but does this mean that public API has been removed?