Use already-running app instances when splitting

This patch makes it so that when the user initiates a split from Home, a running app instance is used instead of always attempting to spawn a new Intent. If no matching app instance is found, behavior is unchanged.

Previously, splitting from Home always resulted in a new Intent being staged, which ignored the fact that an app of the same type could already be running. This resulted in undesirable situations like being able to attempt splitting an app with itself in Overview.

Fixed by querying the RecentsModel when a split of this type is initiated, checking to see if there is a running task of the desired type, and using that to perform the split operation if one is found. When Overview is loaded, applyLoadPlan() will now check to see if there is a staged task, and remove the associated tile as needed. If the removed task is part of a pair, this involves creating a temporary TaskView to hold the other task.

Also fixes a bug where using the Taskbar to select one's second app would incorrectly choose the oldest instance of a multi-instance app rather than the newest.

Fixes: 257513447
Fixes: 259936298
Test: Manual
Change-Id: I97a62f34c03aa4980f9cd743e35e9fc6ef4c092d