Support reviewing history for the logging screen
There's currently no visual change in the logging page when we lose the device connection. We should make sure there is (put a subtle transparent overlay on the logging items?), and do something similar on the other pages as well.
The page footer says "no device connected". Upon losing connection, a toast-like dialog is also shown.
Actually perhaps we should show this UI when we lose device connection? This would be more obvious than the message in the footer.

We do this now, however, users loose all info about the app session (we switch to the connect screen and remove all the other screens). We may want to adjust this as being able to go back and look at data for the app is useful (memory usage? logs info? the timeline info for the last few frames?).
This can be supported by a combination of:
- implementing support for loading offline data for the logging screen: https://github.com/flutter/devtools/issues/4470
- ensuring the logging screen supports reviewing history on disconnect - see this commit for reference.