[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

framework-t/api/system-current.txt

@@ -532,6 +532,7 @@ package android.net.thread {
     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
   }
 

thread/framework/java/android/net/thread/ThreadNetworkException.java

@@ -47,6 +47,7 @@ public class ThreadNetworkException extends Exception {
         ERROR_REJECTED_BY_PEER,
         ERROR_RESPONSE_BAD_FORMAT,
         ERROR_RESOURCE_EXHAUSTED,
+        ERROR_UNKNOWN,
     })
     public @interface ErrorCode {}
 
@@ -122,6 +123,12 @@ public class ThreadNetworkException extends Exception {
      */
     public static final int ERROR_RESOURCE_EXHAUSTED = 10;
 
+    /**
+     * The operation failed because of an unknown error in the system. This typically indicates
+     * that the caller doesn't understand error codes added in newer Android versions.
+     */
+    public static final int ERROR_UNKNOWN = 11;
+
     private final int mErrorCode;
 
     /** Creates a new {@link ThreadNetworkException} object with given error code and message. */