[Large screen] Check if it's in 2-pane before 2-pane deep link flow

The callback of SplitStateObserver.SplitStateListener can have one
or more active splits when there is already 2-pane in Activity stack.

This change prevent unnecessary 2-pane deep link flow if the Activity
is already in 2-pane activity stack.

This solution does not work if the Activity was started in a new task.
(e.g., Intent.FLAG_ACTIVITY_NEW_TASK, launchMode singleTask).

Bug: 201379454
Bug: 201620626
Bug: 204398432
Bug: 204397936
Bug: 197609195
Bug: 197609197
Bug: 204501179
Bug: 204959335
Bug: 204845334
Test: manual
      1. Settings -> Apps > Default apps > Opening links.
      2. Click back button should back to Default apps page.
Change-Id: I04aaceed47a8f2754a4e17c53b49252f61e0a1d1