[ActivityManager] Fix adjust to wrong focused stack

Symptom:
Back from application to home, the previous top activity
of app stack will show awhile then return to home.

Reproduce step:
1.Launch task T from home.
2.Press home key.
3.Launch task A from home.
4.A launches task B.
5.B finish self and A finish onResume.
6.T will show awile and auto return to home.

http://youtu.be/JrFIUHVw4Js

Root Cause:
Not adjust correct focus stack to resume if there is
finishing task.

Solution:
Ignore finishing task when comparing top task.

Change-Id: Id5d6d58ccdac38bdbebba6df50a063b78182cd4f