Add "min size" facility to the Window class.

This allows us to have a new dialog theme that behaves like an alert dialog
for both Dialog and Activity versions.  Very useful with so many more things
being displayed as dialogs on our nice large screen.

Note I didn't change the existing dialog themes to have this behavior, since
it will probably break things.  Instead there is a new variation.  And the
DialogWhenLarge variations now use this for their dialog form, to fix many
of the real new dialogs we have that need this behavior.

Removed the public definition of the one alert dialog theme.  None of the
others have ever been public, this one shouldn't be.

Added new .Panel versions of the Holo themes, like we already had for the
original themes.

Changed the alert dialog layout to no longer use WeightedLinearLayout,
since the window now takes care of that.  This allowed for the removal
of the xlarge version of those layouts.

Change-Id: I0c8372bde25eb9af47404a719b3f07230baf73bf