[CacheService] Auto delete cached items after expiry

[prudrabhat]

Problem

Currently FileCacheService (the default CacheService implementation provided by core) allows components to cache items with an expiry. The cached item is only deleted when the expired item is requested again. The owner of the cached item is currently responsible for clearing these items from the cache on demand otherwise.
While this solution works, it can be improved to delete the content of the cache that have expired. This is especially useful for handling media/images in push notifications and in-app messages. (Imagine several in app message images being cached bloating the app cache)

Proposal

One possible solution is to run a task to delete all the expired content on every app launch. However, handling cache deletion from an SDK should be done in a way not to impact the app resource usage. In order to achieve a balance the following can be done:

• Track the expiry of the cache timestamp with the earliest expiry - scheduledCleanupTimestamp
• When FileCacheService is used to cache an item, check if its expiry is earlier than the currently stored scheduledCleanupTimestamp if yes, overwrite it. Otherwise do nothing
• On each app launch, check scheduledCleanupTimestamp to see if the cleanup job has to be run and run the job.

Some things to take care of:

• Simultaneous access to the cached item (CacheService.get() and delete job happening at once)

Platform and Version

Android 2.x or later.