[android] #5887 - Custom layer crash fix + updated test case

platform/android/MapboxGLAndroidSDK/src/main/java/com/mapbox/mapboxsdk/style/layers/CustomLayer.java

@@ -20,7 +20,7 @@ public class CustomLayer extends Layer {
         super(nativePtr);
     }
 
-    public void invalidate() {
+    public void update() {
         nativeUpdate();
     }
 

platform/android/MapboxGLAndroidSDK/src/main/java/com/mapbox/mapboxsdk/style/layers/Layer.java

@@ -106,7 +106,7 @@ public abstract class Layer {
         }
     }
 
-    public void invalidate() {
+    public final void invalidate() {
         this.invalidated = true;
     }
 }