Class for managing the active scorer application.

Allows listing all possible scorer apps, getting the active scorer
app, setting the active scorer app, and determining whether a given
UID is for the active scorer.

Note that setting a default scorer with a system image will be handled
separately - the plan here is to add a read-only build property
containing a package name to use as the scorer out of the box.
When NetworkScorerService is initialized, it will check a provisioning
bit; if that bit isn't set, it will set the current scorer to the
package specified in this build property (if any) and set the
provisioning bit to true.

Bug: 13769362
Change-Id: I18c0b2ba2aceccc88b70c3611a49149e0bf17ecf