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

quickstep/src/com/android/launcher3/uioverrides/QuickstepLauncher.java

@@ -644,7 +644,7 @@ public class QuickstepLauncher extends Launcher {
                 Collections.singletonList(splitSelectSource.itemInfo.getComponentKey()),
                 false /* findExactPairMatch */,
                 foundTasks -> {
-                    @Nullable Task foundTask = foundTasks.get(0);
+                    @Nullable Task foundTask = foundTasks[0];
                     boolean taskWasFound = foundTask != null;
                     splitSelectSource.alreadyRunningTaskId = taskWasFound
                             ? foundTask.key.id