feat(ngRoute): allow `ngView` to be included in an asynchronously loaded template

During its linking phase, `ngView` relies on the info provided in `$route.current` for
instantiating the initial view. `$route.current` is set in the callback of a listener to
`$locationChangeSuccess`, which is registered during the instantiation of the `$route` service.

Thus, it is crucial that the `$route` service is instantiated _before_ the initial
`$locationChangeSuccess` event is fired. Since `ngView` declares `$route` as a dependency, the
service is instantiated in time, if `ngView` is present during the initial load of the page.

Yet, in cases where `ngView` is included in a template that is loaded asynchronously (e.g. in
another directive's template), the directive factory might not be called soon enough for `$route`
to be instantiated before the initial `$locationChangeSuccess` event is fired.

This commit fixes it, by enabling eager instantiation of `$route` (during the initialization phase).
Eager instantiation can be disabled (restoring the old behavior), but is on by default.

Fixes #1213
Closes #14893

BREAKING CHANGE:

In cases where `ngView` was loaded asynchronously, `$route` (and its dependencies; e.g. `$location`)
might also have been instantiated asynchronously. After this change, `$route` (and its dependencies)
will - by default - be instantiated early on.

Although this is not expected to have unwanted side-effects in normal application bebavior, it may
affect your unit tests: When testing a module that (directly or indirectly) depends on `ngRoute`, a
request will be made for the default route's template. If not properly "trained", `$httpBackend`
will complain about this unexpected request.

You can restore the previous behavior (and avoid unexpected requests in tests), by using
`$routeProvider.eagerInstantiationEnabled(false)`.