Avoid use of covariant return type change fns fixing backwards compat

See, eg, https://jira.mongodb.org/browse/JAVA-2559. This results
in errors like the following:

```
2021-06-12 17:15:58.893 13340-13412/com.example.rnldk E/AndroidRuntime: FATAL EXCEPTION: NioPeerHandler NIO Thread
    Process: com.example.rnldk, PID: 13340
    java.lang.NoSuchMethodError: No virtual method clear()Ljava/nio/ByteBuffer; in class Ljava/nio/ByteBuffer; or its super classes (declaration of 'java.nio.ByteBuffer' appears in /system/framework/core-oj.jar)
        at org.ldk.batteries.NioPeerHandler.lambda$new$0$NioPeerHandler(NioPeerHandler.java:185)
        at org.ldk.batteries.-$$Lambda$NioPeerHandler$f9v_DKRzmlYYXidLTlb3OtA-tVY.run(Unknown Source:2)
        at java.lang.Thread.run(Thread.java:764)
```