Add NETWORK_SETTINGS to SystemApi

The permissions are checked by Tethering; mainline modules should be
able to do permission checks based on these permissions.

Bug: 135411507
Test: m
Test: make test-api-stubs-docs-update-current-api \
      system-api-stubs-docs-update-current-api

Change-Id: Ia617ea56aba18f98371d7dbd546679e0327bfacd