[native] Sort media by modificationTime
[native] Sort media by modificationTime

Summary: I had changed this to `creationTime` due to a user complaint, but today I had trouble finding an image I had just saved. I think `modificationTime` is the better option, and it's more consistent at least on iOS with system behavior and the behavior of popular apps.

Reviewers: palys-swm

Reviewed By: palys-swm

Subscribers: KatPo, zrebcu411, Adrian

Differential Revision: https://phabricator.ashoat.com/D68

Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal