aapt2: Add signature policy to Resources.proto

Adds the new enum values for none and the new signature policy.
None is needed to ensure we don't accidentally deserialize public,
as protobuf uses the first or 0 value enum as the default.

Test: aapt2_tests pass

Change-Id: Ibcb6b961abaa7e5c048e80c4a3eaf1b6d1d421b5