gomobile builds apps with hidden files from the assets directory.
The final applications should skip files like .DS_Store, .gitignore,
etc. Therefore, walker needs to skip the hidden files.
Change-Id: Ibbf7010d525cc831a009f3680f84063f40ac570f
Reviewed-on: https://go-review.googlesource.com/14825
Reviewed-by: David Crawshaw <crawshaw@golang.org>