Check MAINLINE_NETWORK_STACK in ConnectivityService

Treat NetworkStack.PERMISSION_MAINLINE_NETWORK_STACK as equivalent to
NETWORK_STACK, CONNECTIVITY_INTERNAL and NETWORK_SETTINGS combined.
NetworkStack.PERMISSION_MAINLINE_NETWORK_STACK is defined as signature
permission in NetworkStackPermissionStub, which is signed with the
platform certificate, so requirements are the same for both permissions.

The permission is defined by NetworkStackPermissionStub and only used by
NetworkStack, which must be signed with the same key as the stub.

Test: flashed, booted, WiFi works, also with changes on top removing
      CONNECTIVITY_INTERNAL and NETWORK_SETTINGS
Bug: 112869080
Change-Id: I9d0ea8fc99eadf7e902421efdba9b8bf535658b8