[Large sceen] Don't go Settings 2-pane deep link flow if not task root

When a full screen or a non-Settings-app ActivityA starts ActivityB of
a SettingsActivity, splitInfos will be empty and it unexpectedly goes
Settings 2-pane deep link flow.

This change checks if an Activity is not task root, don't go Settings
2-pane deep link flow.

Bug: 205497007
Test: manual
      SUW -> Vision settings
      Check if it goes Settings 2-pane deep link flow.
Change-Id: I0822fbe71fdde47ce7c070986371a4dfea84992c