[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