Populate _template_classes global on first use

This change ensures that the _template_classes global variable is always
initialized when the first template is created. This was done when
instantiating the Template base class, but it was missed when creating an
instance of any of its subclasses.

Change-Id: I5edc9f06d6b48bb04e8ed41365823021ef133909
Closes-Bug: 1397468
Merge "Populate _template_classes global on first use"