Improve user text shown on sync

[lukstbit]

When the user initiates a sync a Snackbar is shown with the text "Collection synchronized" while the media actually goes in the background(there's the notification but that could easily be missed by the user). I feel like it would be better if that text says something about media sync, for example:

Collection sysnchronized. Media changes are synced in the background.

or shorter

Collection sysnchronized. Media is synced in the background.

Originally posted by @lukstbit in https://github.com/ankidroid/Anki-Android/pull/15619#pullrequestreview-1941421935

Note: this is just for discussion, please don't ask to assign the issue yet.