refactor: simplify `SignalInfo` string separator

app/src/main/java/com/geeksville/mesh/ui/SignalInfo.kt

@@ -22,17 +22,16 @@ fun signalInfo(
             nodeInfo.deviceMetrics?.airUtilTx
         )
     } else {
-        buildString {
-            if (nodeInfo.channel > 0) append("ch:${nodeInfo.channel}")
+        buildList {
+            if (nodeInfo.channel > 0) add("ch:${nodeInfo.channel}")
             if (nodeInfo.hopsAway == 0) {
                 if (nodeInfo.snr < 100F && nodeInfo.rssi < 0) {
-                    if (isNotEmpty()) append(" ")
-                    append("RSSI: %d SNR: %.1f".format(nodeInfo.rssi, nodeInfo.snr))
+                    add("RSSI: %d SNR: %.1f".format(nodeInfo.rssi, nodeInfo.snr))
                 }
             } else {
-                append("Hops Away: %d".format(nodeInfo.hopsAway))
+                add("Hops Away: %d".format(nodeInfo.hopsAway))
             }
-        }
+        }.joinToString(" ")
     }
     return if (text.isNotEmpty()) {
         Text(