fix(ngModelOptions): preserve context of getter/setters

Closes #9394

BREAKING CHANGE: previously, ngModel invoked getter/setters in the global context.

For example:

```js
<input ng-model="model.value" ng-model-options="{ getterSetter: true }">
```

would previously invoke `model.value()` in the global context.

Now, ngModel invokes `value` with `model` as the context.

It's unlikely that real apps relied on this behavior. If they did they can use `.bind` to explicilty
bind a getter/getter to the global context, or just reference globals normally without `this`.
It would be nice if `ngModelGet = parsedNgModel` by default, and only when `ctrl.$options` is set and `ctrl.$options.getterSetter` is true do we replace `ngModelGet` with the wrapper handling the getter/setter...