Can we have a Citrine-like app that simply stores all of your pictures, images, music, videos sent to Nostr (NIP-96 Media Servers + Blossom) locally and offer those files to any Nostr client that has the hash via Android inter-app APIs?

Maybe it even syncs with other servers, like Blossom can do?

The goal is to avoid duplicating disk usage between multiple Nostr apps, removing the need for app-centric local caches where the same files are downloaded and stored in every installed Client's local database (like profile pictures). 

 too late. images should've been hash identifiers, not HTTP URLs 

 So...  a local IPFS node. 

 If you can run IPFS at all. IPFS has been notoriously difficult to even run on a phone.  

 In the Unix story we tried to use common shared libraries, common OS installations with a lot of services, to reduce space usage. 
But this method is non so flexible, infact now we use virtual machine, docker container that eats a lot of space but are so flexible.
I don't think reduce disk space sharing the same file is a good idea. 

 It all depends on how many clients are people expected to use. I am expecting that people will use between 30-100 clients if Nostr wins. So, It will take a significant tow on disk space if all the information gets duplicated.