Migrate INetd String16 method to std::string

Test: as follows
    - built
    - flashed
    - booted
    - system/netd/tests/runtests.sh passes
Change-Id: I7fe0e14a23b3c6f82dbfa17c7a44d221720976a0
diff --git a/server/NetdNativeService.h b/server/NetdNativeService.h
index 2f6cb80..c90ff94 100644
--- a/server/NetdNativeService.h
+++ b/server/NetdNativeService.h
@@ -37,7 +37,7 @@
 
     // Firewall commands.
     binder::Status firewallReplaceUidChain(
-            const String16& chainName, bool isWhitelist,
+            const std::string& chainName, bool isWhitelist,
             const std::vector<int32_t>& uids, bool *ret) override;
 
     // Bandwidth control commands.