[Autofill] Fix getAutofillComponent returning null sometimes

Sometimes getAutofillComponent may return null. This can be easily
reproduced by killing the autofill provider, then calling
AutofillManager#getAutofillServiceComponentName, which returns null.
With this change, it'll return the user selected component.

Test: atest CtsAutoFillServiceTestCases
Bug: 319503432
Flag: android.service.autofill.fix_get_autofill_component
Change-Id: I10dd46830feb82c82a8f26084f2fc50685b5a1ec