ManifestProject: add manifest_platform
ManifestProject: add manifest_platform

And fix most of the other attributes to return the value instead of
None.

Change-Id: Iddcbbeb56238ee082bb1cae30adbd27a2f551f3d
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/335134
Tested-by: LaMont Jones <01939f9990a33aa2509cfaed21d2f9ba201a2b37@google.com>
Reviewed-by: Raman Tenneti <66788d848ab03efc83371cd4b586a3110f9172ef@google.com>
Reviewed-by: Xin Li <93465216fd46a440eb04b27d6e009aced8b0c153@google.com>

couchbasedeps/git-repo,lewixliu/git-repo,GerritCodeReview/git-repo,couchbasedeps/git-repo,GerritCodeReview/git-repo,lewixliu/git-repo