[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
(cherry picked from commit 4e32165714cf1b38bec5e819c3c99075f0073566)
Merge pull request #2570 from wuhengzhi/crosswalk-10

[Android] Implement the function for file picker.