Add NonNull annotation to getLanguage implementations

* Add NonNull annotation to getLanguage implementations in LanguageProviderDevice, LanguageProviderAppDefined and LanguageContext classes

Author: tanaynigam

OneSignalSDK/onesignal/src/main/java/com/onesignal/language/LanguageContext.java

@@ -1,5 +1,9 @@
 package com.onesignal.language;
+
+import androidx.annotation.NonNull;
+
 import com.onesignal.OSSharedPreferences;
+
 import static com.onesignal.language.LanguageProviderAppDefined.PREFS_OS_LANGUAGE;
 
 /*
@@ -29,6 +33,7 @@ public void setStrategy(LanguageProvider strategy) {
         this.strategy = strategy;
     }
 
+    @NonNull
     public String getLanguage() {
         return strategy.getLanguage();
     }

OneSignalSDK/onesignal/src/main/java/com/onesignal/language/LanguageProviderAppDefined.java

@@ -1,4 +1,6 @@
 package com.onesignal.language;
+import androidx.annotation.NonNull;
+
 import com.onesignal.OSSharedPreferences;
 
 public class LanguageProviderAppDefined implements LanguageProvider {
@@ -16,6 +18,7 @@ public void setLanguage(String language) {
                 language);
     }
 
+    @NonNull
     public String getLanguage() {
         return preferences.getString(
                 preferences.getPreferencesName(), PREFS_OS_LANGUAGE, "en");

OneSignalSDK/onesignal/src/main/java/com/onesignal/language/LanguageProviderDevice.java

@@ -1,5 +1,7 @@
 package com.onesignal.language;
 
+import androidx.annotation.NonNull;
+
 import java.util.Locale;
 
 public class LanguageProviderDevice implements LanguageProvider {
@@ -17,6 +19,7 @@ public String getLanguage() {
         // https://github.com/OneSignal/OneSignal-Android-SDK/issues/98
         if (lang.equals("zh"))
             return lang + "-" + Locale.getDefault().getCountry();
+    @NonNull
 
         return lang;
     }