Add optional reasons why permissions were denied

Currently only for the ContextImpl.enforcePermission code paths and only
when 'secure debug_package_permission_check' is set.

This feature is disabled on user-builds.

Test: Enabled permission checking for my app and
      - checked runtime permision denial
      - permission denial because app is instant
Bug: 111075456
Change-Id: Ib85777db69ee490608e9dac32a3b97971c0ba215