RCFilters: Reduce startup overhead from 'config.json' computation

RCFilters: Reduce startup overhead from 'config.json' computation

Follows-up 9ba1d8f526e1. Use the new 'versionCallback' to only perform
the minimum amount of work needed to detect changes, without fully
transforming the data for delivery.

Factor out message parsing into a separate step in the tag list code,
and use the messages' raw contents for the versionCallback.

Stop using WANCache for the tag list, since it's now cached by
ResourceLoader, and only regenerated when needed.

Also refactor the ChangeTags functions around tag description messages a
bit, so that we can more easily get the message keys that are going to
be used.

Bug: T201574
Bug: T223260
Change-Id: I02082aeb289ce4156170b14b8840f6d92cbadb57