Keep Settings.Global.NETWORK_RECOMMENDATIONS_ENABLED updated.

Make sure Settings.Global.NETWORK_RECOMMENDATIONS_ENABLED reflects
the current active scorer state. When valid the setting will have a
value of 1, when invalid it will be 0 (or more
accurately not 1, see below).

Introduced a new valid setting value, -1, which indicates the feature
has been forced off.

Test: runtest frameworks-services -c com.android.server.NetworkScorerAppManagerTest
Test: runtest frameworks-services -c com.android.server.NetworkScoreServiceTest
Bug: 35896421
Change-Id: I271725f798e5d0acc7c08c79678dba5115f8faad