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
Something along:
```js
var handledFoo;
$rootScope.foo = 'bar';
$rootScope.$watch('[2, foo, "c"]', function(value) {
    handledFoo = value[1];
}, true);
$rootScope.$digest();
expect(handledFoo).toBe('bar');
$rootScope.foo = 'baz';
$rootScope.$digest();
expect(handledFoo).toBe('baz');
```

I guess other tests can be already checking that.