Allow connectivity-critical packets in data saver mode.

This makes IPv6 work on metered wifi networks. Without this:

1. We reject incoming RAs, so we lose connectivity when the RA
   parameters expire.
2. We reject incoming NAs, so we get NUD failures.

Bug: 66015813
Test: angler builds, boots
Test: netd_{unit,integration}_test pass
Test: CtsHostsideNetworkTests tests pass
Change-Id: I033040ef0b91c22035e29c636123cd41ab1967ec
diff --git a/server/FirewallController.h b/server/FirewallController.h
index 041aa40..1da9e70 100644
--- a/server/FirewallController.h
+++ b/server/FirewallController.h
@@ -64,6 +64,8 @@
 
     int replaceUidChain(const char*, bool, const std::vector<int32_t>&);
 
+    static std::string makeCriticalCommands(IptablesTarget target, const char* chainName);
+
     static const char* TABLE;
 
     static const char* LOCAL_INPUT;