Make NETWORK_RECOMMENDATIONS_ENABLED hidden.

This value is necessary to easily look up the availability of a
recommendation (nee, scoring) provider; however it is only used by
bundled apps to monitor its availability, so hide the setting itself.                               


Test: make update-api; make; flashall
Bug: 34715823

Change-Id: Idf4591fd03d90207ef525f584793db65a1f6597c