isReady() for interstitials & ad expiry

There is currently no way that I know of to determine if an interstitial is loaded, and it's unclear if the expiry times are handled (1 hour expiration on all ads but app open, 4 hour expiration on app open). I could not find anything in the code so there's a decent change expired ads are being shown.

An isReady method should be added for interstitials so it can be determined if preparing again is necessary.

Start by tracing the interstitial preparation and display entry points to determine how loaded ads and expiry times are currently handled. Define completion as exposing an isReady method for interstitials and confirming that the one-hour and four-hour expiry rules are respected without showing expired ads.

Written by the indexing model from the issue text.