Use center hex and grid distance for estimate event

Enumerating all the possible valid hexes for a location estimate can
create extremely large messages for estimates with large radius.

Sending the center hex and grid distance does a few things.
- Compacts the message
- speaks in terms of hexes like most other things in the system
- Provides an easy way to check locations using h3.grid_distance(center, target) <= grid_distance

src/service/poc_mobile.proto

@@ -916,10 +916,12 @@ message rle_event_v1 {
 }
 
 message radio_location_estimate_v1 {
-  // The res12 h3 indexes of the estimated covered area
-  repeated uint64 hexes = 1;
-  Decimal confidence = 4;
-  repeated rle_event_v1 events = 5;
+  // The res12 h3 index representing the center of the estimate
+  uint64 hex = 1;
+  // h3 grid distance the location confidence applies to
+  uint32 grid_distance = 2;
+  Decimal confidence = 3;
+  repeated rle_event_v1 events = 4;
 }
 
 message radio_location_estimates_req_v1 {