Remove duplicate move of home stack that can cause animation jank.

Each move of the home stack in the ActivityManager ends up calling
into the WindowManager which causes layout to be performed.

Also, fixed issue where ActivityStack.moveTaskToFrontLocked()
could move a task to the front without the focus been adjusted to
the new top activity.

Bug: 19692494
Change-Id: Ib4c999c6dfa1af3fda0fced52b58da614b154d00