layout/shell: Keep track of previous role

And with it also handle the re-mapping/unmapping of previous active
window such we don't run into weird behaviours, and be able to revert
back to the original output dimensions when going back to normal
windows.

Bug-AGL: SPEC-4839
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Change-Id: I3703de998b0baa8a2b9d72276dcd56b315d3ca1a