[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
Why onReceiveValue will be called both from `XWalkUIClient.onReceiveValue` and `XWalkViewInternal.onActivityResult`, should be either of them, otherwise, your code should contain logic problem.