[Thread] add error code ERROR_UNKNOWN for forward compatibility
This is per the go/android-api-guidelines#return-error:
```
Make sure your chosen error handling option gives you the
flexibility to introduce new error types in the future. For
@IntDef, that means including an OTHER or UNKNOWN value - when
returning a new code, you can check the caller's targetSdkVersion
to avoid returning an error code the application doesn't know
about. For exceptions, have a common superclass that your
exceptions implement, so that any code that handles that type will
also catch and handle subtypes.
```
Bug: 320403080
Change-Id: I2ee663b75107dac6d84fa697f449fa0424128fde
2 files changed