Add account preference controller.

- Add account preference controller and move the handling for
displaying user's accounts from AccountSettings to the
controller.

- Move AccountPreference and RemoveUserFragment from inner class to
standalone class.

- Add AccountRestrictionHelper to handle API that needs to be mocked for
testing.

Bug: 31801423
Test: make RunSettingsRoboTests
Change-Id: I3d16d0b6a8922a8abec7037c52b7a4de2d76eb0d

src/com/android/settings/dashboard/DashboardDividerDecoration.java

@@ -76,6 +76,9 @@ public class DashboardDividerDecoration extends RecyclerView.ItemDecoration {
         }
         final PreferenceGroupAdapter prefAdapter = (PreferenceGroupAdapter) adapter;
         final int adapterPosition = parent.getChildAdapterPosition(view);
+        if (adapterPosition == RecyclerView.NO_POSITION) {
+            return false;
+        }
         final Preference pref = prefAdapter.getItem(adapterPosition);
         final Preference nextPref = prefAdapter.getItem(adapterPosition + 1);
         if (nextPref == null) {