In a PWA, the loaderServicePromise cannot be reused across page
navigations; style[amp-extension="amp-loader"] is lost. Add a dispose
method to reset the promise when shadowDoc.close() runs.

This reverts commit 7253153.

Cached LoaderService becomes invalid on page transition in a PWA. Rely
on .installExtensionForDoc() to short-circuit quickly when the service
is already available and to install the doc extension when needed.