feat(input): constant exprs for  ngTrueValue/ngFalseValue

If ngTrueValue / ngFalseValue are strings evaluating to constant expressions, 
the constant value is used in place of the string value.

If the value is not a constant, its string value is used, as they were
previously.

If no string expression is provided, then the default value remains true or
false.

Fixes #1199