Changed Autofill Save UI to show UPDATE instead of SAVE when necessary.

Also introduced new metric: FIELD_AUTOFILL_UPDATE on AUTOFILL_SAVE_UI.

Test: atest CtsAutoFillServiceTestCases # with a bunch of changes
Test: atest OptionalSaveActivityTest#testShowUpdateWhenUserChangedOptionalValueFromDatasetAndRequiredNotFromDataset

Bug: 112192360
Fixes: 80100983

Change-Id: Ifab0d43de44fd473a3f8b232d77d5dc62344b0ae