Post onProvideAutofillStructure in Augment Autofill flow to UI thread

This is to fix the issue that onProvideAutofillStructure is called on the binder thread causing CtsInputMethodTestCases FocusHandlingTest#testOnCheckIsTextEditorRunOnUIThreadWithInputMethodManagerIsActive to fail.

Bug: b/362908997
Change-Id: Ic68dca1e0baef2e5b00c248c4417b8891749cdcf
Test: atest
CtsAutoFillServiceTestCases:android.autofillservice.cts.augmented
Flag: EXEMPT bugfix