[ios] Fixed hang observing user defaults on background queue

Fixed a deadlock that could occur if the host application attempts to set user defaults on a background queue. Now we observer user defaults changes synchronously.

Cherry-picked from d76ef6e53b.

platform/ios/CHANGELOG.md

@@ -2,6 +2,10 @@
 
 Mapbox welcomes participation and contributions from everyone.  If you’d like to do so please see [CONTRIBUTING.md](https://github.com/mapbox/mapbox-gl-native/blob/master/CONTRIBUTING.md) first to get started.
 
+## 3.2.1
+
+- Fixed a hang that could occur if the host application attempts to set user defaults on a background queue. ([#4745](https://github.com/mapbox/mapbox-gl-native/pull/4745))
+
 ## 3.2.0
 
 - If you’ve previously installed the SDK as a static framework, the installation workflow has changed to address issues when submitting your application to the App Store or installing it on a device. Upon upgrading to this version of the SDK, you’ll need to add Mapbox.bundle to the Copy Bundle Resources build phase and remove Mapbox.framework from the Embed Frameworks build phase. ([#4455](https://github.com/mapbox/mapbox-gl-native/pull/4455))

platform/ios/src/MGLMapboxEvents.m

@@ -128,7 +128,6 @@ const NSTimeInterval MGLFlushInterval = 180;
 @end
 
 @implementation MGLMapboxEvents {
-    id _userDefaultsObserver;
     NSString *_instanceID;
 }
 
@@ -195,17 +194,8 @@ const NSTimeInterval MGLFlushInterval = 180;
             self.canEnableDebugLogging = YES;
         }
         
-
         // Watch for changes to telemetry settings by the user
-        __weak MGLMapboxEvents *weakSelf = self;
-        _userDefaultsObserver = [[NSNotificationCenter defaultCenter] addObserverForName:NSUserDefaultsDidChangeNotification
-                                                                                  object:nil
-                                                                                   queue:[NSOperationQueue mainQueue]
-                                                                              usingBlock:
-         ^(NSNotification *notification) {
-             MGLMapboxEvents *strongSelf = weakSelf;
-             [strongSelf pauseOrResumeMetricsCollectionIfRequired];
-         }];
+        [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(userDefaultsDidChange:) name:NSUserDefaultsDidChangeNotification object:nil];
        
         [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(pauseOrResumeMetricsCollectionIfRequired) name:UIApplicationDidEnterBackgroundNotification object:nil];
         [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(pauseOrResumeMetricsCollectionIfRequired) name:UIApplicationWillEnterForegroundNotification object:nil];
@@ -229,7 +219,7 @@ const NSTimeInterval MGLFlushInterval = 180;
 }
 
 - (void)dealloc {
-    [[NSNotificationCenter defaultCenter] removeObserver:_userDefaultsObserver];
+    [[NSNotificationCenter defaultCenter] removeObserver:self];
     [self pauseMetricsCollection];
 }
 
@@ -245,6 +235,10 @@ const NSTimeInterval MGLFlushInterval = 180;
     return _instanceID;
 }
 
+- (void)userDefaultsDidChange:(NSNotification *)notification {
+    [self pauseOrResumeMetricsCollectionIfRequired];
+}
+
 - (void)pauseOrResumeMetricsCollectionIfRequired {
     // Prevent blue status bar when host app has `when in use` permission only and it is not in foreground
     if ([CLLocationManager authorizationStatus] == kCLAuthorizationStatusAuthorizedWhenInUse &&