[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
diff --git a/framework-t/api/system-current.txt b/framework-t/api/system-current.txt
index 3513573..d346af3 100644
--- a/framework-t/api/system-current.txt
+++ b/framework-t/api/system-current.txt
@@ -532,6 +532,7 @@
field public static final int ERROR_RESPONSE_BAD_FORMAT = 9; // 0x9
field public static final int ERROR_TIMEOUT = 3; // 0x3
field public static final int ERROR_UNAVAILABLE = 4; // 0x4
+ field public static final int ERROR_UNKNOWN = 11; // 0xb
field public static final int ERROR_UNSUPPORTED_CHANNEL = 7; // 0x7
}