Add new flag to control the refactor of policy transparency

The new API for policy transparency and the usage of it will be controlled with this flag. As it's only a refactoring which won't introduce user visible change, it's not considered a feature. That's why we use PURPOSE_BUGFIX tag to enable faster rollout.

Flag: android.app.admin.flags.policy_transparency_refactor_enabled
Bug: 414733570

Change-Id: Ic554d32d38d953e0ec9fb8b5684d183b2746810d