Add SELinux checks when adding a service.

Add a MAC check to the svc_can_register function in
service_manager. The types are defined in
external/sepolicy/service.te and the mapping from service
names is defined in external/sepolicy/service_contexts.
Currently uses the property context backend to parse the
contexts file.

Bug: 12909011
Change-Id: I5d90a614263c60571c7c70c2882e6fa929911ca5