subtitle downlaoder: retain local copy on iOS for local media

When downloading subtitles for locally stored media, retain the copy. For remote content and on tvOS, continue to store in the caches folder so it is eventually removed. Fixes #892
5 jobs for !244 with retainosodownloads in 24 minutes and 15 seconds (queued for 12 minutes and 9 seconds)
detached
Status Job ID Name Coverage
  Lint
passed #453015
macos
lint

00:05:28

 
  Build
passed #453084
macos
iOS 12.0

00:10:03

passed #453085
macos
tvOS 12.0

00:08:43

failed #453016
macos
iOS 12.0

00:08:07

failed #453017
macos
tvOS 12.0

00:07:26