java.lang.NoClassDefFoundError: java.nio.charset.StandardCharsets Crash

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- [ ] Bug exists Release Version 1.2.0 ( Master Branch)
- [x] Bug exists in MQTTv3 Client on Snapshot Version 1.2.1-SNAPSHOT (Develop Branch)
- [ ] Bug exists in MQTTv5 Client on Snapshot Version 1.2.1-SNAPSHOT (Develop Branch)

If this is a bug regarding the Android Service, please raise the bug here instead: https://github.com/eclipse/paho.mqtt.android/issues/new

When I use Android 4.1, Paho Mqtt Client's version is 1.2.1, crash

I see org.eclipse.paho.client.mqttv3.internal.wire.MqttWireMessage have these code:

protected static final Charset STRING_ENCODING = StandardCharsets.UTF_8;

I find the StandardCharsets support min Android API is 19 (Android 4.4),
so how to solve this problem when i use Paho Mqtt Version 1.2.1 and Android 's version is Android 4.0 or up