[webapp-container] AP tests crash when state saver is enabled

Based on the non yet lander branch:

https://code.launchpad.net/~abreu-alexandre/webbrowser-app/remove-session-storage

that disables the session storage bits from the container, when subsequently enabling the StateSaver to take over part of the work done by the session storage, AP tests produce a crash when being run on the device.

In order to repro it:

- take the branch above (unless it has landed in the meantime),
- add

StateSaver.properties: "url"
StateSaver.enabled: true

to src/app/webcontainer/WebApp.qml ,

- compile on the device & run the AP tests for the container on the device

autopilot3 run webapp_container

there should be a crash file in /var/crash.

TO BE NOTED:
----------------------

- The tests actually succeed, the crash happens as the app is being closed,
- The stack trace (at least for me) was corrupted,
- There seem to be a race somewhere since, adding a bit of delay in any AP test that maked the container crash (even a sleep of 1sec) makes the issue go away,
- The state saver does quite a bit in terms of binding to the app lifecycle, creating an archive for the state backups, etc. as the app is being launched and does some things also (cleanup archive file, save archive file, etc.) as the app is being closed, and might end up in a messy state since the tests do involve a very quick run of the app,