[snap] Upgrade from deb to snap forgets saved passwords

On Ubuntu 19.10, when upgrading from the deb package to the snap¹, the password-manager-service interface is not auto-connected. As a result, chromium is unable to talk to org.freedesktop.Secret.Service over D-Bus, and it falls back to the basic unencrypted stored.

See https://forum.snapcraft.io/t/auto-connecting-the-cups-control-and-password-manager-service-interfaces-for-the-chromium-snap/4592 for why auto-connection was denied for the password-manager-service interface.

This is bad user experience, as the transition should be seamless.

Aside of auto-connection, one possible solution would be to do the connection in the deb’s pre-install script (which installs the snap from the store, and is executed as root). That has pretty much the same security implications though, so it requires input from the security team before proceeding.

Yet another solution would be for the wrapper script (/usr/bin/chromium-browser) to extract the stored passwords from the keyring and import them into the basic unencrypted sqlite store for the snap, but that feels clunky and fragile.

Or a log message / dialog box at startup to inform the user that they should connect the interface manually. But that's not great UX, and doesn't provide a seamless transition.

¹ https://discourse.ubuntu.com/t/call-for-testing-chromium-browser-deb-to-snap-transition/11179