Support custom push notification icon in Xamarin android.

This brings us up to parity with the main Parse android SDK: https://parse.com/tutorials/android-push-notifications.

Fixes #39.

Author: richardjrossiii

Parse/Internal/ManifestInfo.cs

@@ -107,6 +107,19 @@ public static int IconId {
         }
         return iconId;
       }
+    }
+
+    private static int pushIconId = -1;
+    public static int PushIconId {
+      get {
+        lock (mutex) {
+          if (pushIconId == -1) {
+            var customIcon = GetApplicationMetaData().GetInt("com.parse.push.notification_icon", -1);
+            pushIconId = customIcon == -1 ? IconId : customIcon;
+          }
+          return pushIconId;
+        }
+      }
     }
 
     private static Intent launcherIntent;

Parse/ParsePush.Android.cs

@@ -44,8 +44,8 @@ public static void DefaultParsePushNotificationReceivedHandler(object sender, Pa
       NotificationCompat.Builder builder = new NotificationCompat.Builder(context)
         .SetContentTitle(new Java.Lang.String(title))
         .SetContentText(new Java.Lang.String(alert))
-        .SetTicker(new Java.Lang.String(tickerText))
-        .SetSmallIcon(ManifestInfo.IconId)
+        .SetTicker(new Java.Lang.String(tickerText))
+        .SetSmallIcon(ManifestInfo.PushIconId)
         .SetContentIntent(pContentIntent)
         .SetAutoCancel(true)
         .SetDefaults(NotificationDefaults.All);