Add setLanguage to Android plugin

* Add setLanguage method to Android plugin in OneSignalPlugin.java

Author: tanaynigam

android/src/main/java/com/onesignal/flutter/OneSignalPlugin.java

@@ -120,6 +120,8 @@ else if (call.method.contentEquals("OneSignal#setExternalUserId"))
       this.setExternalUserId(call, result);
     else if (call.method.contentEquals("OneSignal#removeExternalUserId"))
       this.removeExternalUserId(result);
+    else if (call.method.contentEquals("OneSignal#setLanguage"))
+      this.setLanguage(call, result);
     else if (call.method.contentEquals("OneSignal#initNotificationOpenedHandlerParams"))
       this.initNotificationOpenedHandlerParams();
     else if (call.method.contentEquals("OneSignal#initInAppMessageClickedHandlerParams"))
@@ -337,6 +339,14 @@ public void onFailure(OneSignal.OSSMSUpdateError error) {
     });
   }
 
+  private void setLanguage(MethodCall call, final Result result) {
+    String language = call.argument("language");
+    if(language != null && language.length() == 0)
+      language = null;
+
+      OneSignal.setLanguage(language);
+  }
+
   private void setExternalUserId(MethodCall call, final Result result) {
     String externalUserId = call.argument("externalUserId");
     String authHashToken = call.argument("authHashToken");