Check if last resumed activity is null before resuming.

Previous implementation did not encounter this issue as the resumed
activity was simply assigned to the stack member variable. Since we
now rely on a callback from the activity record, we must check to
see if the record is null before invoking #setState on it.

Test: atest CtsAutoFillServiceTestCases:android.autofillservice.cts.SessionLifecycleTest#testDatasetVisibleWhileAutofilledAppIsLifecycled
Bug: 74604879
Change-Id: Icead81d44a826260145f72549a9caa0fb62d6af3