LayoutLib: fix the background of title/action bars.

The bars are supposed to sit on top of the window so
that if they are transparent (which is the default case
for the action bar), the window background shows through.

Fix this using a layout to represent the window content
(all but the system bar) in which the title/action bars
and the layout content sits. This layout receives the
windowBackground drawable.

Change-Id: I8072c5ff52f585fa32acb589d8526e1c13cbcd94