Always look for MODE_OPENING during task launch from overview

* Previously the target mode was dependent on whether liveTile
was running or not, opposite targets seem to be coming in from
WM now.
* Remove a bunch of un-used code from LauncherSplitScreenListener
that was used when we only allowed one split pair in overview
previously. Now that is handled by RecentTasksController in shell
so we don't need any additional bookkeeping.

Fix: 212220301
Test: Able to launch side tasks in overview
Change-Id: I002340d98ba379cb4ab9ae37eadfa235d6b5e56d