[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
As the design, the APIs in XWalkUIClientInternal was exposed to external users by comparison to ```XWalkWebChromeClient```. In android webview, ```onShowFilePicker()``` was located in ```android.webkit.WebChromeClient```, so I move it to ```XWalkWebChromeClient``` with the discussion with @wang16.
If the user has the requisition to expose it, we will move it back.