Fix crash in TriangleCustomLayerActivity (#10399)

### Summary

Fixes https://mapbox.atlassian.net/browse/MAPSAND-2488

- Fixes a crash in `TriangleCustomLayerActivity` caused by using a
removed/incompatible layer insertion API.

### Problem

`TriangleCustomLayerActivity` crashed when adding a custom layer because
it used `addLayerBelow(CustomLayer(...), below = "building")`, which
depends on a specific layer ID (`"building"`) that may not be present in
the current style, causing a runtime crash.

### Solution

Replace `addLayerBelow` with `addLayer` using the slot-based API
(`slot("middle")`), which is the correct modern approach for inserting
custom layers at a predefined position in the style without hardcoding a
layer ID dependency.

### Key Changes

- **`TriangleCustomLayerActivity.kt`**: `addLayerBelow(CustomLayer(...),
below = "building")` → `addLayer(customLayer(...) { slot("middle") })`
- **Import**: Removed `CustomLayer` and `addLayerBelow` imports; now
uses `addLayer` and the `customLayer` DSL builder

### Validation

- [x] Activity no longer crashes when loading
- [x] Verify the custom triangle layer renders correctly at the expected
z-order in the `middle` slot

GitOrigin-RevId: 7de2a5108b26d665c4c5091bd120808a01f8ab45

Author: pengdev

app/src/main/java/com/mapbox/maps/testapp/examples/customlayer/TriangleCustomLayerActivity.kt

@@ -8,8 +8,7 @@ import com.mapbox.geojson.Point
 import com.mapbox.maps.CameraOptions
 import com.mapbox.maps.MapboxMap
 import com.mapbox.maps.Style
-import com.mapbox.maps.extension.style.layers.CustomLayer
-import com.mapbox.maps.extension.style.layers.addLayerBelow
+import com.mapbox.maps.extension.style.layers.addLayer
 import com.mapbox.maps.extension.style.layers.customLayer
 import com.mapbox.maps.extension.style.layers.properties.generated.ProjectionName
 import com.mapbox.maps.extension.style.projection.generated.projection
@@ -45,9 +44,10 @@ class TriangleCustomLayerActivity : AppCompatActivity() {
   }
 
   private fun addCustomLayer(style: Style) {
-    style.addLayerBelow(
-      CustomLayer(CUSTOM_LAYER_ID, TriangleCustomLayer()),
-      below = "building"
+    style.addLayer(
+      customLayer(CUSTOM_LAYER_ID, TriangleCustomLayer()) {
+        slot("middle")
+      }
     )
     binding.fab.setImageResource(R.drawable.ic_layers_clear)
   }