Update to consistently highlight selection for single field.

Confirmed with UX that:
1. Highlight all fields filled by Autofill.
2. Do NOT highlight for Augmented Autofill.

We will stop hiding the highlight in normal Autofill session, but still keep hiding the highlight for Augmented Autofill: https://source.corp.google.com/h/googleplex-android/platform/frameworks/base/+/main:core/java/android/service/autofill/augmented/AugmentedAutofillService.java;l=496.

With that saying, we will keep the flags PFLAG4_AUTOFILL_HIDE_HIGHLIGHT and its related helper functions.

Local test:
Before, single field, no highlight: https://screenshot.googleplex.com/7E9wSTdD8cM4Fiy
Before, multi fields, with highlight: https://screenshot.googleplex.com/4QdUueUJYTPShin
After, single field, with highlight: https://screenshot.googleplex.com/6oEmeKnugVZCbhm
After, multi fields, with highlight: https://screenshot.googleplex.com/98MezWKRm745yX5
After, Augmented Autofill, Smart Reply, suggested:
https://screenshot.googleplex.com/4tXMp4W8VFiVKJS
After, Augmented Autofill, Smart Reply, no highlight: https://screenshot.googleplex.com/5KnUvN5jR6WmKqC

Flag: android.service.autofill.highlight_autofill_single_field
Bug:b/41496744
Test: atest android.autofillservice.cts.LoginWithCustomHighlightActivityTest

Change-Id: Ie56586634ad8836695d4c8a26282cb1375f49113