resourceloader: Avoid unnecessary xmlns attributes in ResourceLoaderImage

resourceloader: Avoid unnecessary xmlns attributes in ResourceLoaderImage

We were creating the `<g>` element without specifying a namespace,
which caused the library to add `xmlns` attributes with the document's
default SVG namespace to elements that we appended underneath it.
(At least, that's what I think was happening.)

Specify the SVG namespace when creating it to avoid the mess and
reduce resulting file size.

Change-Id: Ida27494aeae9dece16f878c16cf9aa582e6deac3