Use the NETWORK_STACK permission instead of null when registering
broadcast receivers in ConnectivityService, PermissionMonitor and
BroadcastReceiveHelper.
This ensures that only components with the NETWORK_STACK permission
can send these protected broadcasts, improving system security.
Tests were updated to reflect the new permission requirement.
Flag: EXEMPT trivial change
Test: atest ConnectivityCoverageTests:android.net.connectivity.com.android.server.connectivity.BroadcastReceiveHelperTest \
ConnectivityCoverageTests:android.net.connectivity.com.android.server.connectivity.PermissionMonitorTest \
--update-device
Fix: 425828059
Change-Id: I9761f9f454751761b346e255aac0d1c7d7a7e30c
ca5a062321