Fix NetworkTemplate generates non-metered carrier template

From current design, NetworkPolicyManagerService should only
creates metered carrier/mobile templates. However, if someone
calls the hidden API interface or the template was created
before NetworkTemplate#Builder is published. The caller may
create a non-metered carrier template and persist into the
storage.

This CL elimates this possibility and mark non-metered carrier
template non-persistable, so devices could auto-recover from
this symptom after reboot.

Fix: 222382637
Test: TH
Change-Id: Ia7ba3d92a84b8b246a4f11080f6fec0957865650