Fixed Cancellation for PendingAutofillRequest.

1. mCallback.cancel() now correctly cancels the pending request.
2. AutofillProxy.update() now updates the CancellationSignal.

Fixes: 132459064
Test: atest android.autofillservice.cts.augmented
Change-Id: If8b2ab5c0fd8db4cfad440bf47031f6d85c28ae5
Merged-In: If8b2ab5c0fd8db4cfad440bf47031f6d85c28ae5