Document and refactor root-level services

[matanlurey]

As part of the runApp migration, I ran into the following issues:

• You could never override services in bootstrapFactory (missing feature)
• To be able to override services, I add to add a hacky "middle" injector (internal):

/// **INTERNAL ONLY**: Creates a new application-level Injector.
///
/// This is more complicated than just creating a new Injector, because we want
/// to make sure we allow [userProvidedInjector] to override _some_ top-level
/// services (`APP_ID`, `ExceptionHandler`) _and_ to ensure that Angular-level
/// services (`ApplicationRef`) get the user-provided versions.
Injector _appInjector(InjectorFactory userProvidedInjector) {
  // These are the required root services.
  final minimalInjector = minimalApp(platformInjector());

  // These are the user-provided overrides.
  final userInjector = userProvidedInjector(minimalInjector);

  // Get a handle of NgZone, so we can create ApplicationRef in it.
  final NgZone ngZone = unsafeCast(minimalInjector.get(NgZone));

  // ... and then we add ApplicationRef, which has the unique property of
  // injecting services (specifically, `ExceptionHandler` and `APP_ID`) that
  // might have come from the user-provided injector, instead of the minimal.
  //
  // We also add other top-level services with similar constraints:
  // * `AppViewUtils`
  return ngZone.run(() {
    return new Injector.map({
      ApplicationRef: new ApplicationRefImpl(
        unsafeCast(minimalInjector.get(PlatformRef)),
        ngZone,
        userInjector,
      ),
      AppViewUtils: new AppViewUtils(
        unsafeCast(userInjector.get(APP_ID)),
        unsafeCast(minimalInjector.get(SanitizationService)),
        unsafeCast(minimalInjector.get(EventManager)),
      ),
    }, userInjector);
  });
}

---

... which led me to the following:

• We should document what root-level services are provided by AngularDart
• We should document what can be overridden and what cannot
  • AFAIK, only ExceptionHandler and APP_ID are really safe for overriding
• We should consider removing the root-level injector entirely, making DI pseudo optional

The reason I say pseudo optional is that likely we will still need to provide some services, likely through an Injector.map, like above, so others can inject them in DI, but we don't need to use DI ourselves in order to create the root-level services.