Bring back the priority feature for static RRO

Supporting the priority feature is requested by partners because they
want to layer RRO for their customers.
Applying the priority feature on non-static RRO isn't feasible because
their priority should be managed directly by users.

Test: building succeeded and tested on sailfish device.
Bug: 37185740
Change-Id: I8ef83c5e3178bd395ec7e6498764a535f24ef235