Fix AmbientDisplayAlwaysOn isSliceable.

I changed this method to return true in ag/22761786 thinking that it was
necessary (as per the example at
https://g3doc.corp.google.com/company/teams/apps-android-settings/howto/settings_slices.md#preference-controller),
but this breaks a test, so I'm rolling back that part of the change.

Fix: 279881487
Test: presubmit
Change-Id: I87716a0daf5face9a79f98353b49ea2166226279