Android: add xamarin build type

Adding xamarin build type, which is essentially
a clone of unity except it requires 4 properties to
be passed to gradle upon invocation:
- enableXamarin
- manifestApplicationId
- onesignal_app_id
- onesignal_google_project_number

e.g., gradle assemble [ -Pproperty=value .. ]

Author: Vivek Ayer

OneSignalSDK/onesignal/build.gradle

@@ -19,6 +19,17 @@ android {
         unity {
             minifyEnabled false
         }
+
+        xamarin {
+            minifyEnabled false
+            if (project.hasProperty('enableXamarin') && enableXamarin == 'true') {
+                if (project.hasProperty('manifestApplicationId') && project.hasProperty('onesignal_app_id') && project.hasProperty('onesignal_google_project_number')) {
+                    manifestPlaceholders = [manifestApplicationId:manifestApplicationId, onesignal_app_id:onesignal_app_id, onesignal_google_project_number:onesignal_google_project_number]
+                } else {
+                    throw new GradleException('Please ensure you have provided manifestApplicationId, onesignal_app_id, and onesignal_google_project_number! aborting xamarin build!')
+                }
+            }
+        }
     }
 }
 
@@ -29,4 +40,4 @@ dependencies {
     compile 'com.google.android.gms:play-services-location:8.4.0'
 }
 
-apply from: 'maven-push.gradle'
+apply from: 'maven-push.gradle'

OneSignalSDK/onesignal/src/xamarin/AndroidManifest.xml

@@ -0,0 +1,21 @@
+<manifest xmlns:android="http://schemas.android.com/apk/res/android" package="com.onesignal">
+
+    <!-- Create a unique permission for your app and use it so only your app can receive your OneSignal messages. -->
+    <permission android:name="${manifestApplicationId}.permission.C2D_MESSAGE" android:protectionLevel="signature" />
+    <uses-permission android:name="${manifestApplicationId}.permission.C2D_MESSAGE" />
+
+    <application>
+        <meta-data android:name="onesignal_app_id" android:value="${onesignal_app_id}" />
+        <meta-data android:name="onesignal_google_project_number" android:value="str:${onesignal_google_project_number}" />
+
+        <receiver
+            android:name="com.onesignal.GcmBroadcastReceiver"
+            android:permission="com.google.android.c2dm.permission.SEND" >
+            <intent-filter>
+                <action android:name="com.google.android.c2dm.intent.RECEIVE" />
+                <category android:name="${manifestApplicationId}" />
+            </intent-filter>
+        </receiver>
+    </application>
+
+</manifest>