Update flatbuffers to 1.9.0 (#20186)

* Update flatbuffers to 1.9.0

This fix updates flatbuffers to 1.9.0. The previous version
used (971a681) in tf was released last year, and is not
a versioned release. This fix updates to the latest versioned
release of 1.9.0.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Add missing files of java_generator.cc to fix build error.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>