[mobile] Increase stream response buffer watermark size to 2MB (#32754)

Signed-off-by: Renjie Tang <renjietang@google.com>

mobile/library/common/http/client.cc

@@ -121,10 +121,10 @@ void Client::DirectStreamCallbacks::encodeData(Buffer::Instance& data, bool end_
     response_data_ = std::make_unique<Buffer::WatermarkBuffer>(
         [this]() -> void { onBufferedDataDrained(); }, [this]() -> void { onHasBufferedData(); },
         []() -> void {});
-    // Default to 1M per stream. This is fairly arbitrary and will result in
+    // Default to 2M per stream. This is fairly arbitrary and will result in
     // Envoy buffering up to 1M + flow-control-window for HTTP/2 and HTTP/3,
-    // and having local data of 1M + kernel-buffer-limit for HTTP/1.1
-    response_data_->setWatermarks(1000000);
+    // and having local data of 2M + kernel-buffer-limit for HTTP/1.1
+    response_data_->setWatermarks(2 * 1024 * 1024);
   }
 
   // Send data if in default flow control mode, or if resumeData has been called in explicit