[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
Ok, after reading a bit into the code, what you should do is
1. Leave the API in XWalkUIClient unchanged.
2. In `XWalkContentClientBridge.shouldOverrideRunFileChooser`, when `syncNullReceived`, return `XWalkView.showFileChooser()`
3. in `XWalkView.showFileChooser()`, put your implementation here.