Non colliding positioning of entering resizeable tasks.

When a new task enters freeform work space, it becomes full screen. This
CL makes it non fullscreen upon entry and also tries to position the
task in a way, that won't collide with existing tasks (so it's clear to
the user, that they are still there).

Change-Id: Ia04fdcadd0e85b268c323358cd4b3e04affa7939