[applets/systemtray] Add workaround for Dropbox

System Tray forgets Dropbox hidden state after logout+login

The SNI Id should be consistent between sessions, but Dropbox does not
follow the SNI specification - it adds PID as a suffix of Id.

As this is very popular application and it was not fixed upstream for
many years it is reasonable to add workaround in SystemTray.

BUG: 378910
FIXED-IN: 5.22.0