make permission_cache_key_for shard aware

fixes: CNVS-13917

Moves the initializer from active record base to an alias method chain
on AdheresToPolicy::Instance methods.  The way its including the
instance methods did not allow it to override the
permission_cache_key_for method.

Test Case:

  - Flush cache
  - Open a Course
  - Look at the cache keys and all the ids should be global ids

Change-Id: Iac6a5ed95a800c27bd53fcb757cabd11976aef21
Reviewed-on: https://gerrit.instructure.com/37226
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>
Product-Review: Nick Cloward <ncloward@instructure.com>