Fix "null" toast when FillCallback.onFailure(null) called.

Root cause
The error message is set by AutofillService, AutofillService calls
FillCallback.onFailure(null) back to the platform. Because a081250
uses String.valueOf(message), this method will make a null object
become to a "null" string. This may cause the Autofill session to
think we should show message because of a non-null message. And if
the AutofillService target SDK is lower than 29, we don't ignore show
message, then users will see a "null" toast.

Solution
Instead of passing the message parameter to String.valueOf() directly
, we check the message first, only passing to String.valueOf() if the
message is not null. And we pass an empty string if the message
parameter is null.

Bug: 159888723
Test: Use a test AutofillService which target SDK is lower than 29,
make sure no null toast is shown
Test: atest CtsAutoFillServiceTestCases

Change-Id: Id7ff581b3a7f35cf97b4f3405a40bfa1a420823e
(cherry picked from commit 3c7ac7de46)

services/autofill/java/com/android/server/autofill/RemoteFillService.java

@@ -161,8 +161,9 @@ final class RemoteFillService extends ServiceConnector.Impl<IAutoFillService> {
 
                 @Override
                 public void onFailure(int requestId, CharSequence message) {
+                    String errorMessage = message == null ? "" : String.valueOf(message);
                     fillRequest.completeExceptionally(
-                            new RuntimeException(String.valueOf(message)));
+                            new RuntimeException(errorMessage));
                 }
             });
             return fillRequest;