Manifest merger v2 - Basic implementation: It can process all the specia...

...l tags in the manifest

I would like to add this as an *experimental* feature for now and later add more checks and feature from the manifest-update mojo if possible.

I prefer to keep it separate from the manifest-update mojo because we might not have the same features in the end.

We also need to tackle the sourceManifestFile, androidManifestFile, updatedManifestFile in another pull request.