[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
`return mXWalkView.showFileChooser(uploadFile, acceptTypes, Boolean.toString(capture));` and make `XWalkViewInternal.showFileChooser` return boolean type.