Icon, text or whatever indicator to see if sync is working

After the bug #1547 resolution (the app wasn't sync at all, and now it's working) it will be very great to have something (icon, text, whatever indicator) to show us if sync is working or not !

### What
An icon, or a text, or a button in the app to show us if the application is sync with the server.
Sync : OK/waiting/down

### Why
This indicator will be more efficient for us to check the sync, and prevent logout when not working.

### How
Put the indicator where you want ;)

The issue names no files, tests, or entry points; start by tracing the Android app’s existing sync behavior and locating its user-facing status area. Define the visible OK, waiting, and down states, then verify that the indicator reflects the server sync state and is visible in the app.

Written by the indexing model from the issue text.