Allow better customization of Autofill dialogs.

1. Allow Autofill Service to use a custom icon/header for SaveUI
2. Allow Save/Fill Dialog icon to be hidden

Fix: 242703084
Test: manual:
 * atest android.autofillservice.cts.saveui
 * atest android.autofillservice.cts.dialog.LoginActivityTest
 * manually check that custom icon/header is shown/hidden

Change-Id: Ic4bb6964a8e8355b7b2a191d810143745dd95059