GoogleARCoreJavaHelper functions are not correctly exported publicly

The jni functions in GoogleARCoreAndroidHelper.cpp are not correctly exported, when we tried to build a Test project it failed with this exception:

 java.lang.UnsatisfiedLinkError: No implementation found for void com.google.arcore.unreal.GoogleARCoreJavaHelper.onApplicationCreated() (tried Java_com_google_arcore_unreal_GoogleARCoreJavaHelper_onApplicationCreated and Java_com_google_arcore_unreal_GoogleARCoreJavaHelper_onApplicationCreated__)

Adding JNI_METHOD in front of them fixed the issue on our branch. This is an easy fix, might be good for 4.21

1. Create android project using GoogleARCore
2. Build a Test configuration or other config where it doesn't export symbols by default
3. Executable will fail to find the GoogleARCoreJavaHelper functions