Support custom themes in TabHost.

CL 76118 introduced style customized tab layout. However, applications
which tabWidgetStyle has been overriden and does not inherit from
Widget.TabWidget will not be able to access tabLayout. Use the default
holo layout for these.

Bug 3235944

Change-Id: I5666c5ef2886060a2175a3d642b012cdaf0a1d50