Move or refactor PermissionManagerInternal APIs.

Make the three backup related methods in PermissionManagerInternal
ready for system API. PermissionManagerInternal is currently used in
framework so it can't be removed without other changes yet. The other
listener methods are only used by PermissionPolicyService and will
become module internal.

Finally turn PermissionManagerInternal and
PermissionManagerServiceInternal into an interface from an abstract
class, and remove redundant modifiers after this.

Also refactors onUserCreated/Removed() to be system API ready.

Bug: 158736025
Test: presubmit
Change-Id: I335a7a37b737f6fa0faf0a2c34634d44199aee97