[Android] Implement the function for file picker.

This patch is to implement the callback about file picker with
customized java code. The show of file picker has no relationship
with the java code in Chromium.

BUG=XWALK-2631, XWALK-2717
Merge pull request #2553 from wuhengzhi/file_picker_not_work1

[Android] Implement the function for file picker.