Preserve window during resize triggered relaunches.

This changes application code behavior when the activity relaunches due
to configuration change. It only applies to scenarios, where the
configuration change was triggered by a user generated resize of an
activity (i.e. user drags a corner of an activity and thus changes its
size).

Preserving a window means that we will keep the decor view and non
client decor view around, but remove all children views when the
activity gets destroyed. When the activity gets created again, it will
attach its new content to the preserved view hierarchy. Mind, we
actually recreate application side Window object, since some of its
features might changed, but we retain its elevation (to not trigger
relayout with new layout params).

Preserving the window also means that we don't call the window manager
service to remove and later add the window. Instead, we continue using a
single window state throughout the resize operation.

Change-Id: Ie3d2878ed09c99ff343044bfe7a29a0ba07a265e