Work around Map field in proto

Map in proto3, specifically in Q throws build errors. Since we will
backport to Q, we can create our own Map object to work around this.

Bug: 148217342
Test: mma
Change-Id: I759e240beeee2c2a4413f3e5236d0feab901bf72
diff --git a/car-messenger-common/proto/notification_msg.proto b/car-messenger-common/proto/notification_msg.proto
index 447fed6..6329d04 100644
--- a/car-messenger-common/proto/notification_msg.proto
+++ b/car-messenger-common/proto/notification_msg.proto
@@ -137,10 +137,15 @@
   string notification_key = 1;
 
   //Optional, used to capture data like the reply string.
-  map<string, string> action_data = 2;
+  repeated ActionDataFieldEntry action_data_field = 2;
 
   ActionName action_name = 3;
 
   // Unique id of this action.
   int64 request_id = 4;
 }
+
+message ActionDataFieldEntry {
+  string key = 1;
+  string value = 2;
+}