Coalesce duplicated method GetTimeSinceMeshPacket (#4968)

GetTimeSinceMeshPacket was duplicated in PowerTelemetry and
EnvironmentalTelemetry, albeit one had a cooler name than the other.

As we add HealthTelemetry, to avoid creating a third instance of
this method, let's move it somewhere that makese sense.

Adds a new method GetTimeSinceMeshPacket to MeshService and updates
EnvironmentTelemetry and PowerTelemetry to use it.

src/mesh/MeshService.h

@@ -151,6 +151,8 @@ class MeshService
 
     ErrorCode sendQueueStatusToPhone(const meshtastic_QueueStatus &qs, ErrorCode res, uint32_t mesh_packet_id);
 
+    uint32_t GetTimeSinceMeshPacket(const meshtastic_MeshPacket *mp);
+
   private:
 #if HAS_GPS
     /// Called when our gps position has changed - updates nodedb and sends Location message out into the mesh
@@ -163,4 +165,4 @@ class MeshService
     friend class RoutingModule;
 };
 
-extern MeshService *service;
+extern MeshService *service;