WifiManager.getCountryCode(): Change permission to NETWORK_SETTINGS

Previously, the documented permission was
CONNECTIVITY_INTERNAL, which has been deprecated.
The permission that was actually being checked was
NETWORK_STACK. However, since this API is only
called by Settings, NETWORK_SETTINGS is a more
appropriate permission.

Bug: 151091657
Test: compiles
Test: Start Soft AP
Change-Id: Ib68ed714471dbbb0367a1e3852144fb1affbccb0