fix(elements): correctly handle getting/setting properties before connecting the element (#36114)

`createCustomElements()` creates some getters/setters for properties
corresponding to component inputs that delegate to the
`NgElementStrategy`. However, it is not guaranteed that the element's
`NgElementStrategy` will have been created when these getters/setters
are called, because some polyfills (e.g. `document-register-element`) do
not call the constructor.

Previously, trying to get/set input properties before connecting the
element to the DOM (via `connectedCallback()`) would fail due to
`NgElementStrategy` not being created.

This commit ensures that the `NgElementStrategy` is always created
before used inside the input property getters/setters (similar to how it
is done for other methods of `NgElement`).

Mentioned in https://github.com/angular/angular/pull/31416/files#r300326698.

PR Close #36114