Make PhabricatorPolicyInterface require a getPHID() method

Summary:
Ref T603. This cleans up an existing callsite in the policy filter, and opens up some stuff in the future.

Some policy objects don't have real PHIDs:

  PhabricatorTokenGiven
  PhabricatorSavedQuery
  PhabricatorNamedQuery
  PhrequentUserTime
  PhabricatorFlag
  PhabricatorDaemonLog
  PhabricatorConduitMethodCallLog
  ConduitAPIMethod
  PhabricatorChatLogEvent
  PhabricatorChatLogChannel

Although it would be reasonable to add real PHIDs to some of these (like `ChatLogChannel`), it probably doesn't make much sense for others (`DaemonLog`, `MethodCallLog`). Just let them return `null`.

Also remove some duplicate `$id` and `$phid` properties. These are declared on `PhabricatorLiskDAO` and do not need to be redeclared.

Test Plan: Ran the `testEverythingImplemented` unit test, which verifies that all classes conform to the interface.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D7306