Define permission for system apps to query size of block

Allows ManagedProvisioning to determine whether there's a
challenge and thus whether to disable NFC provisioning.

Other implementation option: new hidden boolean API method.
Can't think of benefit of new API method "isBlockInUse", other
than doesn't leak PDB size and is more explicitly tied to the
use case. Open to either impl if anyone has opinions on the matter.

Bug: 18508767
Change-Id: I28d2eb5a0837ff85cb91f140b17ce1dd843fe9d6