Add GcmPushHelper class with a few helper functions to get tokens and sendId

app/src/main/java/chat/rocket/android/push/gcm/GcmPushHelper.kt

+package chat.rocket.android.push.gcm
+
+import bolts.Task
+import chat.rocket.android.R
+import chat.rocket.android.RocketChatApplication
+import chat.rocket.android.RocketChatCache
+import chat.rocket.android.api.RaixPushHelper
+import chat.rocket.persistence.realm.RealmHelper
+import chat.rocket.persistence.realm.models.ddp.RealmUser
+import com.google.android.gms.gcm.GoogleCloudMessaging
+import com.google.android.gms.iid.InstanceID
+import java.io.IOException
+
+object GcmPushHelper {
+
+    fun getGcmToken(): String? = getGcmToken(getSenderId())
+
+    @Throws(IOException::class)
+    private fun registerGcmTokenForServer(realmHelper: RealmHelper): Task<Void> {
+        val gcmToken = getGcmToken(getSenderId())
+        val currentUser = realmHelper.executeTransactionForRead({ realm -> RealmUser.queryCurrentUser(realm).findFirst() })
+        val userId = if (currentUser != null) currentUser.getId() else null
+        val pushId = RocketChatCache.getOrCreatePushId()
+
+        return RaixPushHelper(realmHelper)
+                .pushUpdate(pushId!!, gcmToken, userId)
+    }
+
+    @Throws(IOException::class)
+    private fun getGcmToken(senderId: String): String {
+        return InstanceID.getInstance(RocketChatApplication.getInstance())
+                .getToken(senderId, GoogleCloudMessaging.INSTANCE_ID_SCOPE, null)
+    }
+
+    private fun getSenderId(): String {
+        return RocketChatApplication.getInstance().getString(R.string.gcm_sender_id)
+    }
+}
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