FW : Screen Sizing for TabUI when HiddenNav Enabled

Hidden Nav is not available to Tablets, but it could be
selected in RC for Phablet/Phone.  If UI then switched
to Tablet, PhoneWindowManager was calculating window sizes
as if NavHide was on.

Change-Id: Ib700fc944dcfb0ad42e7c651ee415f539ed5d134
Signed-off-by: Zaphod <mwilson@acesappraisal.com>