Change HeuristicsService to always provide a hint.

This is helpful to help app developers visualize when autofill is triggered
when it shouldn't (for example, in a chat conversation window), so they can
mark the root view of such activities with
android:importantForAutofill=noExcludeDescendants

Bug: 114236837
Test: manual verification

Change-Id: I33e18e0ed6a274d7918b363e03f90f7decd902b9