Create new window-side task on demand.

The activity manager can create a new task under certain
circumstances in resetTargetTaskIfNeededLocked(). In that case the
window manager was not told to create a new task. This fix creates the
task in window manager when it recognizes that there is a new task
required.

Fixes bug 10412588.

Change-Id: Ife9ecfeb7f53f9df5950322c9359c836adc258a2