Make persistent_data_block_service a system_api_service.

Settings needs to be able to access it when opening developer options.

Address the following denial:
avc:  denied  { find } for service=persistent_data_block scontext=u:r:system_app:s0 tcontext=u:object_r:persistent_data_block_service:s0 tclass=service_manager

Bug: 20131472
Change-Id: I85e2334a92d5b8e23d0a75312c9b4b5bf6aadb0b