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.cpp

@@ -406,4 +406,16 @@ int MeshService::onGPSChanged(const meshtastic::GPSStatus *newStatus)
 bool MeshService::isToPhoneQueueEmpty()
 {
     return toPhoneQueue.isEmpty();
-}
+}
+
+uint32_t MeshService::GetTimeSinceMeshPacket(const meshtastic_MeshPacket *mp)
+{
+    uint32_t now = getTime();
+
+    uint32_t last_seen = mp->rx_time;
+    int delta = (int)(now - last_seen);
+    if (delta < 0) // our clock must be slightly off still - not set from GPS yet
+        delta = 0;
+
+    return delta;
+}