Security-related cleanup for network scoring.

-Perform additional checks for the SCORE_NETWORKS permission when
broadcasting scoring requests to the active scorer and when accepting
score updates. In theory, these checks are unnecessary as we manually
check package manager when obtaining the list of valid scorers, but
they cannot hurt to add.

-Fix multi-user. Since the active scorer is a global setting, we
ensure that scoring can only be done by apps available to the primary
user / owner of the phone, and that the request scores broadcast is
sent to that user's profile. When the scorer is changed, we send that
to all user profiles as it's just informational, although it's
unlikely that apps outside the primary user's profile would need to
respond.

Bug: 14117916
Bug: 16399238
Change-Id: Iaf06bda244eec730b590a30a3f4ffab4965bde96