Style fixes in the external pref64 support.

- Address a code review comment to pass a variable by reference
  instead of by pointer.
- Add two missed EXCLUDES() thread-safety annotations.
- Make "dumpsys dnsresolver" report externally-set prefixes
  differently from discovered prefixes.

Bug: 156914456
Test: existing unit tests pass
Original-Change: https://android-review.googlesource.com/1316178
Merged-In: Ifcdb16cc66c9234a2bd5f8aac70be809fe0dd223
Change-Id: Ifcdb16cc66c9234a2bd5f8aac70be809fe0dd223
diff --git a/Dns64Configuration.h b/Dns64Configuration.h
index c06a6c1..387de78 100644
--- a/Dns64Configuration.h
+++ b/Dns64Configuration.h
@@ -82,8 +82,8 @@
     void stopPrefixDiscovery(unsigned netId);
     netdutils::IPPrefix getPrefix64(unsigned netId) const;
 
-    int setPrefix64(unsigned netId, const netdutils::IPPrefix* pfx);
-    int clearPrefix64(unsigned netId);
+    int setPrefix64(unsigned netId, const netdutils::IPPrefix& pfx) EXCLUDES(mMutex);
+    int clearPrefix64(unsigned netId) EXCLUDES(mMutex);
 
     void dump(netdutils::DumpWriter& dw, unsigned netId);