Add java.nio.ByteOrder as to well-known immutable list

[PhilippWendler]

Description of the problem / feature request:

Please add java.nio.ByteOrder to the list of known immutable types in WellknownMutability.

The class is effectively an enum, although not implemented as such. It is final and does not expose any mutable fields.

Feature requests: what underlying problem are you trying to solve with this feature?

We are using the existing class as an enum for modeling byte orders and would like to store it as a field in other @Immutable classes.

What version of Error Prone are you using?

2.4.0

[PhilippWendler]

Thanks, that was fast!