Implement PushTask to get backends from server

Author: kunall17

app/src/main/java/com/zulip/android/networking/AsyncGetBackends.java

@@ -0,0 +1,15 @@
+package com.zulip.android.networking;
+
+import com.zulip.android.ZulipApp;
+
+public class AsyncGetBackends extends ZulipAsyncPushTask {
+
+    public AsyncGetBackends(ZulipApp app) {
+        super(app);
+    }
+
+    //Json Format - {"msg":"","password":false,"google":false,"result":"success","dev":true}
+    public final void execute() {
+        execute("POST", "v1/get_auth_backends");
+    }
+}