ResourcesManager: fix lookups with ThemeConfig

Always include a theme config in resource keys, regardless of whether
it is deemed as themeable or not.

This prevents creating additional asset managers for each top level
resource call. Instead it will cache them properly.

Change-Id: I687311275d62773460e7771a2e223b75ff94ed8f
Signed-off-by: Roman Birg <roman@cyngn.com>