Migrate null annotations to AndroidX for tests

* Roboelectric 4.3 depended on the Android Support Library but since we
upgraded to 4.4 it has move to AndroidX so these importeds need to be
updated.

Author: jkasten2

OneSignalSDK/unittest/src/test/java/com/onesignal/MockOSSharedPreferences.java

@@ -1,7 +1,7 @@
 package com.onesignal;
 
-import android.support.annotation.NonNull;
-import android.support.annotation.Nullable;
+import androidx.annotation.NonNull;
+import androidx.annotation.Nullable;
 
 import java.util.HashMap;
 import java.util.Set;

OneSignalSDK/unittest/src/test/java/com/onesignal/MockOneSignalAPIClient.java

@@ -1,6 +1,6 @@
 package com.onesignal;
 
-import android.support.annotation.NonNull;
+import androidx.annotation.NonNull;
 
 import org.json.JSONObject;
 

OneSignalSDK/unittest/src/test/java/com/onesignal/ShadowHmsInstanceId.java

@@ -1,7 +1,8 @@
 package com.onesignal;
 
 import android.content.Context;
-import android.support.annotation.Nullable;
+
+import androidx.annotation.Nullable;
 
 import com.huawei.hms.aaid.HmsInstanceId;
 import com.huawei.hms.common.ApiException;