Workaround to display menus for AppCompat themes

Layoutlib currently does not implement menus for the support Toolbar.
When it detects that an AppCompat theme is being used, it will try to
use by default the support Toolbar which breaks the display of menus.
This works around the problem by forcing layoutlib
to use the framework toolbar in menu previews (regular activities
won't display the menu if they use AppCompat).

Bug: http://b.android.com/210946
Change-Id: Ic1d162c6d74119ef42895776c3bc3851a9549120