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/BandwidthController.h b/server/BandwidthController.h
index 45535bb..c8dad99 100644
--- a/server/BandwidthController.h
+++ b/server/BandwidthController.h
@@ -86,6 +86,8 @@
     enum IptFailureLog { IptFailShow, IptFailHide = IptFailShow };
 #endif
 
+    std::string makeDataSaverCommand(IptablesTarget target, bool enable);
+
     int manipulateSpecialApps(const std::vector<std::string>& appStrUids, const std::string& chain,
                               IptJumpOp jumpHandling, IptOp appOp);