Ensure task will be attached when starting activity from AppTask

ActivityManager#addAppTask can add a non-attached task into recents.
And then the caller can use ActivityManager.AppTask#startActivity
to launch activity in the task.

The specified task will be mInTask, but before putting activity in it,
the task must attach to hierarchy, otherwise it may cause NPE on random
places by the incomplete launch.

Bug: 227634021
Test: atest ActivityStarterTests#testStartActivityInner_inTask
Change-Id: I66242e2ffa94be19bad49354b5d9cccded17b25c