[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
When ```onReceiveValue``` was called from ```XWalkUIClient```, ```completed``` was set to ```true```. ```onReceiveValue``` was called again from ```XWalkViewInternal.onActivityResult``` with result. It will throw ```IllegalStateException```.