Add Settings.Global.NETWORK_RECOMMENDATIONS_ENABLED flag.

This flag enables the NetworkScoreService.requestRecommendations API,
defined in b/32909424.

Bug: 32913919
Bug: 32909424
Test: runtest --path
frameworks/base/packages/SettingsProvider/test/src/com/android/providers/settings/SettingsProviderTest.java

Change-Id: If592e4a2ec574929aa0c829c39ad4ce9b0317805