[android] Check if Mapbox instance exists before creating default file source

This check is needed for mbgl-benchmark target, as it does not require Mapbox
instance, nor Android asset manager.

platform/android/src/mapbox.cpp

@@ -9,6 +9,12 @@ jni::Local<jni::Object<AssetManager>> Mapbox::getAssetManager(jni::JNIEnv& env)
     return javaClass.Call(env, method);
 }
 
+jboolean Mapbox::hasInstance(jni::JNIEnv& env) {
+    static auto& javaClass = jni::Class<Mapbox>::Singleton(env);
+    auto method = javaClass.GetStaticMethod<jboolean()>(env, "hasInstance");
+    return javaClass.Call(env, method);
+}
+
 void Mapbox::registerNative(jni::JNIEnv& env) {
     jni::Class<Mapbox>::Singleton(env);
 }