ios: add a CallKit icon

The SDK will now search for an asset called "CallKitIcon" on the main bundle,
and fallback to a built-in asset it it's not there, allowing SDK users to
customize it by just adding asset with that name.

ios/app/src/Images.xcassets/CallKitIcon.imageset/Contents.json

@@ -0,0 +1,23 @@
+{
+  "images" : [
+    {
+      "idiom" : "universal",
+      "filename" : "CallKit@1x.png",
+      "scale" : "1x"
+    },
+    {
+      "idiom" : "universal",
+      "filename" : "CallKit@2x.png",
+      "scale" : "2x"
+    },
+    {
+      "idiom" : "universal",
+      "filename" : "CallKit@3x.png",
+      "scale" : "3x"
+    }
+  ],
+  "info" : {
+    "version" : 1,
+    "author" : "xcode"
+  }
+}

ios/sdk/src/callkit/CallKit.m

@@ -206,14 +206,20 @@ RCT_EXPORT_METHOD(updateCall:(NSString *)callUUID
     // iconTemplateImageData
     NSString *iconTemplateImageName = dictionary[@"iconTemplateImageName"];
     NSData *iconTemplateImageData;
+    UIImage *iconTemplateImage;
     if (iconTemplateImageName) {
-        UIImage *iconTemplateImage
-            = [UIImage imageNamed:iconTemplateImageName
-                         inBundle:[NSBundle bundleForClass:self.class]
-    compatibleWithTraitCollection:nil];
+        // First try to load the resource from the main bundle.
+        iconTemplateImage = [UIImage imageNamed:iconTemplateImageName];
+
+        // If that didn't work, use the one built-in.
+        if (!iconTemplateImage) {
+            iconTemplateImage = [UIImage imageNamed:iconTemplateImageName
+                                           inBundle:[NSBundle bundleForClass:self.class]
+                      compatibleWithTraitCollection:nil];
+        }
+
         if (iconTemplateImage) {
-            iconTemplateImageData
-                = UIImagePNGRepresentation(iconTemplateImage);
+            iconTemplateImageData = UIImagePNGRepresentation(iconTemplateImage);
         }
     }