Delay removal of windows from WindowManager

When relaunching activities the window manager clears out all windows
by calling a ViewRootImpl.die() in a deferred fashion. Then it
immediately deletes the ViewRootImpl and its view from its list
of windows. When the die() is eventually executed it calls
dispatchDetachedFromWindow() which tries to remove the previously
removed windows causing an Exception to be thrown.

This change waits to remove the windows until after die() has been
completed. Fixes bug 8253030.

Change-Id: I5b41be1c6b776e32128c064267653db98bd95292