Return normalized IPv4 addresses from grpc_sockaddr_to_uri().

Users of the high-level API should not care whether we're using AF_INET
or AF_INET6 sockets internally.
diff --git a/src/core/iomgr/sockaddr_utils.c b/src/core/iomgr/sockaddr_utils.c
index 71ac12e..65ec1f9 100644
--- a/src/core/iomgr/sockaddr_utils.c
+++ b/src/core/iomgr/sockaddr_utils.c
@@ -170,6 +170,11 @@
 char *grpc_sockaddr_to_uri(const struct sockaddr *addr) {
   char *temp;
   char *result;
+  struct sockaddr_in addr_normalized;
+
+  if (grpc_sockaddr_is_v4mapped(addr, &addr_normalized)) {
+    addr = (const struct sockaddr *)&addr_normalized;
+  }
 
   switch (addr->sa_family) {
     case AF_INET: