Expose network scoring APIs to /system apps.

Ideally, we'd only expose the methods that we intend unbundled apps to
call (e.g. not NetworkScoreManager#setActiveScorer, which should only
be called by Settings), but this isn't harmful in terms of permissions
as the APIs still check security appropriately.

Bug: 15833200
Change-Id: I2047515b41c8be0cf7cb51dd495fe72309c05f68