Replace direct download link with bazel mirror (mirror.bazel.build) (#19713)

* Replace direct download link with bazel mirror (mirror.bazel.build)

Since the download package for gemmlowp has been propagated
to the bazel mirror (mirror.bazel.build), this fix replaced the
direct link with the mirrored one, and removed the related TODO.

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

* Remove TODO in tensorflow/contrib/lite/download_dependencies.sh

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