[ios] Simplified MBXAnnotationView

Eliminated the center view in favor of applying a background color and border to the annotation view’s layer.

platform/ios/app/MBXAnnotationView.h

@@ -1,7 +1,4 @@
 #import <Mapbox/Mapbox.h>
 
 @interface MBXAnnotationView : MGLAnnotationView
-
-@property (nonatomic) UIColor *centerColor;
-
 @end

platform/ios/app/MBXAnnotationView.m

@@ -1,28 +1,16 @@
 #import "MBXAnnotationView.h"
 
 @interface MBXAnnotationView ()
-
-@property (nonatomic) UIView *centerView;
-
 @end
 
 @implementation MBXAnnotationView
 
 - (void)layoutSubviews {
     [super layoutSubviews];
-    if (!self.centerView) {
-        self.backgroundColor = [UIColor blueColor];
-        self.centerView = [[UIView alloc] initWithFrame:CGRectInset(self.bounds, 1.0, 1.0)];
-        self.centerView.backgroundColor = self.centerColor;
-        [self addSubview:self.centerView];
-    }
-}
-
-- (void)setCenterColor:(UIColor *)centerColor {
-    if (![_centerColor isEqual:centerColor]) {
-        _centerColor = centerColor;
-        self.centerView.backgroundColor = centerColor;
-    }
+    
+    self.layer.borderColor = [UIColor blueColor].CGColor;
+    self.layer.borderWidth = 1;
+    self.layer.cornerRadius = 2;
 }
 
 - (void)setSelected:(BOOL)selected animated:(BOOL)animated

platform/ios/app/MBXViewController.m

@@ -587,22 +587,20 @@ static NSString * const MBXViewControllerAnnotationViewReuseIdentifer = @"MBXVie
     {
         annotationView = [[MBXAnnotationView alloc] initWithReuseIdentifier:MBXViewControllerAnnotationViewReuseIdentifer];
         annotationView.frame = CGRectMake(0, 0, 10, 10);
-        annotationView.centerColor = [UIColor whiteColor];
-       
+        annotationView.backgroundColor = [UIColor whiteColor];
+        
         // uncomment to make the annotation view draggable
         // also note that having two long press gesture recognizers on overlapping views (`self.view` & `annotationView`) will cause weird behaviour
         // comment out the pin dropping functionality in the handleLongPress: method in this class to make draggable annotation views play nice
         annotationView.draggable = YES;
-
-       
+        
         // uncomment to force annotation view to maintain a constant size when the map is tilted
         // by default, annotation views will shrink and grow as the move towards and away from the
         // horizon. Relatedly, annotations backed by GL sprites ONLY scale with viewing distance currently.
         // annotationView.scalesWithViewingDistance = NO;
-        
     } else {
         // orange indicates that the annotation view was reused
-        annotationView.centerColor = [UIColor orangeColor];
+        annotationView.backgroundColor = [UIColor orangeColor];
     }
     return annotationView;
 }