feat(input): support constant expressions for ngTrueValue/ngFalseValue

ngTrueValue and ngFalseValue now support parsed expressions which the parser determines to be constant values.

BREAKING CHANGE:

Previously, these attributes would always be treated as strings. However, they are now parsed as
expressions, and will throw if an expression is non-constant.

To convert non-constant strings into constant expressions, simply wrap them in an extra pair of quotes, like so:

    <input type="checkbox" ng-model="..." ng-true-value="'truthyValue'">