[breaking] remove deprecated v1 embedding references

* Remove references to the now deprecated v1 Android embedding. `PluginRegistry.Registrar` and `io.flutter.view.FlutterNativeView` classes so that the SDK can be used on newer Flutter versions.
* Most imports were unused and leftover from when v2 support was added.
* However, we now need to completely drop v1 support.

Author: nan-li

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

@@ -15,8 +15,6 @@
 import io.flutter.plugin.common.MethodChannel;
 import io.flutter.plugin.common.MethodChannel.MethodCallHandler;
 import io.flutter.plugin.common.MethodChannel.Result;
-import io.flutter.plugin.common.PluginRegistry;
-import io.flutter.plugin.common.PluginRegistry.Registrar;
 
 public class OneSignalDebug extends FlutterRegistrarResponder implements MethodCallHandler {
 

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

@@ -21,7 +21,6 @@
 import io.flutter.plugin.common.MethodChannel;
 import io.flutter.plugin.common.MethodChannel.MethodCallHandler;
 import io.flutter.plugin.common.MethodChannel.Result;
-import io.flutter.plugin.common.PluginRegistry.Registrar;
 
 public class OneSignalInAppMessages extends FlutterRegistrarResponder implements MethodCallHandler, 
 IInAppMessageClickListener, IInAppMessageLifecycleListener{

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

@@ -15,8 +15,6 @@
 import io.flutter.plugin.common.MethodChannel;
 import io.flutter.plugin.common.MethodChannel.MethodCallHandler;
 import io.flutter.plugin.common.MethodChannel.Result;
-import io.flutter.plugin.common.PluginRegistry;
-import io.flutter.plugin.common.PluginRegistry.Registrar;
 
 public class OneSignalLocation extends FlutterRegistrarResponder implements MethodCallHandler {
 

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

@@ -33,8 +33,6 @@
 import io.flutter.plugin.common.MethodChannel;
 import io.flutter.plugin.common.MethodChannel.MethodCallHandler;
 import io.flutter.plugin.common.MethodChannel.Result;
-import io.flutter.plugin.common.PluginRegistry;
-import io.flutter.plugin.common.PluginRegistry.Registrar;
 
 public class OneSignalNotifications extends FlutterRegistrarResponder implements MethodCallHandler, INotificationClickListener, INotificationLifecycleListener, IPermissionObserver {
     private final HashMap<String, INotificationWillDisplayEvent> notificationOnWillDisplayEventCache = new HashMap<>();

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

@@ -24,9 +24,6 @@
 import io.flutter.plugin.common.MethodChannel;
 import io.flutter.plugin.common.MethodChannel.MethodCallHandler;
 import io.flutter.plugin.common.MethodChannel.Result;
-import io.flutter.plugin.common.PluginRegistry;
-import io.flutter.plugin.common.PluginRegistry.Registrar;
-import io.flutter.view.FlutterNativeView;
 
 /** OnesignalPlugin */
 public class OneSignalPlugin extends FlutterRegistrarResponder implements FlutterPlugin, MethodCallHandler, ActivityAware {
@@ -87,24 +84,6 @@ public void onReattachedToActivityForConfigChanges(@NonNull ActivityPluginBindin
   public void onDetachedFromActivityForConfigChanges() {
   }
 
-  // This static method is only to remain compatible with apps that don’t use the v2 Android embedding.
-  @Deprecated()
-  @SuppressLint("Registrar")
-  public static void registerWith(Registrar registrar) {
-    final OneSignalPlugin plugin = new OneSignalPlugin();
-    plugin.init(registrar.activeContext(), registrar.messenger());
-
-    // Create a callback for the flutterRegistrar to connect the applications onDestroy
-    registrar.addViewDestroyListener(new PluginRegistry.ViewDestroyListener() {
-      @Override
-      public boolean onViewDestroy(FlutterNativeView flutterNativeView) {
-        // Remove all handlers so they aren't triggered with wrong context
-        plugin.onDetachedFromEngine();
-        return false;
-      }
-    });
-  }
-
   @Override
   public void onMethodCall(MethodCall call, Result result) {
     if (call.method.contentEquals("OneSignal#initialize"))

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

@@ -21,8 +21,6 @@
 import io.flutter.plugin.common.MethodChannel;
 import io.flutter.plugin.common.MethodChannel.MethodCallHandler;
 import io.flutter.plugin.common.MethodChannel.Result;
-import io.flutter.plugin.common.PluginRegistry;
-import io.flutter.plugin.common.PluginRegistry.Registrar;
 
 public class OneSignalPushSubscription extends FlutterRegistrarResponder implements MethodCallHandler, IPushSubscriptionObserver {
 

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

@@ -10,8 +10,6 @@
 import io.flutter.plugin.common.MethodChannel;
 import io.flutter.plugin.common.MethodChannel.MethodCallHandler;
 import io.flutter.plugin.common.MethodChannel.Result;
-import io.flutter.plugin.common.PluginRegistry;
-import io.flutter.plugin.common.PluginRegistry.Registrar;
 
 public class OneSignalSession extends FlutterRegistrarResponder implements MethodCallHandler {
 

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

@@ -18,8 +18,6 @@
 import io.flutter.plugin.common.MethodChannel;
 import io.flutter.plugin.common.MethodChannel.MethodCallHandler;
 import io.flutter.plugin.common.MethodChannel.Result;
-import io.flutter.plugin.common.PluginRegistry;
-import io.flutter.plugin.common.PluginRegistry.Registrar;
 
 public class OneSignalUser extends FlutterRegistrarResponder implements MethodCallHandler, IUserStateObserver {