Fixes two bugs with findLastActiveTasksAndRunCallback()

This CL changes findLastActiveTasksAndRunCallback to return a fixed array rather than an ArrayList.

1) This fixes a (minor) crash with app pairs, where we performed get(0) on an empty array when no running tasks were found.
2) This also fixes a live bug where app pairs would launch apps in the wrong order if app 1 was not found in running tasks, but app 2 was.

The function should be more robust now, preserving the indexing of the input keys.

Fixes: 316053131
Test: Clear all tasks from Overview, launch an app pair, launch succeeds
Flag: ACONFIG com.android.wm.shell.enable_app_pairs DEVELOPMENT
Change-Id: I51d8ba823a2ec57e9ecfeede956e0afce1d653f4