Allow USE_RESTRICTED_NETWORKS for registerSystemDefaultNetworkCallback

Some callers (e.g. iwlan) need to learn about system default
network but they cannot have NETWORK_SETTINGS permission.
To allow them to use this API but prevent from misuse by
unprivileged apps, enforce USE_RESTRICTED_NETWORKS for this API.

Test: atest com.android.server.ConnectivityServiceTest#testRegisterPrivilegedDefaultCallbacksRequireNetworkSettings android.net.cts.ConnectivityManagerTest#testRegisterNetworkCallback
Fix: 242456635
Change-Id: I80ab27445af874328c9c0f4814a8fbf035ae5df4