Do not attach the added recent task to TDA

An empty task was attached to a TaskDisplayArea during
ActivityManager#addAppTask() and the empty task was detected
by tests as a leaked task.

Bug: 177475203
Test: run sample program to invoke ActivityManager#addAppTask()
Change-Id: Ibe09d7f0381705defe2fc82bf11339da2aaa6294
Merged-In: Ibe09d7f0381705defe2fc82bf11339da2aaa6294