fix(locators): use the root element only to find the testability API, not to scope searches

In 9a8f45af49633f1637c88960ba079d7d425ca72c a change was introduced which made
Protractor's custom locators (by.binding, by.model, etc) use `config.rootElement`
as the root for all their searches. This meant that `config.rootElement` was used
both to specify how to get hold of Angular's injector as well as where to begin
searching for elements. This does not work for all cases, for example if a dialog
should be searched for elements but is a sibling, not a child, of `ng-app`.

This reverts that change, and uses `document` as the parent for all searches. This
is consistent with the behavior of the native locators by.id, by.css, and friends,
which do not scope their search based on `config.rootElement`.