Refactor alert notification logic

- Change `notificationLightColor` to be lazy initialized.
- Update alert notification to use `CATEGORY_ALARM`.
- Use `dataPacket.alert` instead of `dataPacket.text` for alert content.
- Add `alert` property to `DataPacket` to handle alert messages.

app/src/main/java/com/geeksville/mesh/DataPacket.kt

@@ -85,6 +85,13 @@ data class DataPacket(
             null
         }
 
+    val alert: String?
+        get() = if (dataType == Portnums.PortNum.ALERT_APP_VALUE) {
+            bytes?.decodeToString()
+        } else {
+            null
+        }
+
     constructor(to: String?, channel: Int, waypoint: MeshProtos.Waypoint) : this(
         to = to,
         bytes = waypoint.toByteArray(),

app/src/main/java/com/geeksville/mesh/service/MeshService.kt

@@ -305,7 +305,7 @@ class MeshService : Service(), Logging {
         serviceNotifications.showAlertNotification(
             contactKey,
             getSenderName(dataPacket),
-            dataPacket.text ?: getString(R.string.critical_alert)
+            dataPacket.alert ?: getString(R.string.critical_alert)
         )
     }
 

app/src/main/java/com/geeksville/mesh/service/MeshServiceNotifications.kt

@@ -44,10 +44,12 @@ class MeshServiceNotifications(
     private val context: Context
 ) {
 
-    val notificationLightColor = if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
-        context.getColor(R.color.colorPrimary)
-    } else {
-        Color.GREEN
+    val notificationLightColor by lazy {
+        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
+            context.getColor(R.color.colorPrimary)
+        } else {
+            Color.GREEN
+        }
     }
 
     companion object {
@@ -361,7 +363,7 @@ class MeshServiceNotifications(
         with(alertNotificationBuilder) {
             setContentIntent(openMessageIntent(contactKey))
             priority = NotificationCompat.PRIORITY_HIGH
-            setCategory(Notification.CATEGORY_MESSAGE)
+            setCategory(Notification.CATEGORY_ALARM)
             setAutoCancel(true)
             setStyle(
                 NotificationCompat.MessagingStyle(person)