fix($parse): handle constants as one-time binding expressions

Handle constant expressions as one-time binding expressions.
Avoids the infinite digest from
https://github.com/angular/angular.js/pull/7960/files#r14136938
@mzgol you cannot watch a non-constant literal expression without specifying `objectEquality = true` without triggering a digest issue. This was the case before the one-time binding patch and now