Change SessionStore so it doesn't violate disk avoidence goal

As another data point, following on legacy/trac#28507 (moved), we're also violating the disk avoidance goal because Fennec saves information about open tabs (and recently closed tabs) on disk in case the app is killed (see the SessionStore). This allows for restoring the browser state when the app is next opened.

I hesitate saying we should rip this out because if we want a usable browser on Android, then we'll need something like this. If the app doesn't save state, then when the user switches to another app Android may kill the browser (because it is now running in the background) and the user will lose their entire browser session.

Given the available options, I think we should keep the SessionStore enabled and rely on the legacy/trac#28507 (moved) preferences for clearing saved state when the user explicitly closes the app.

I think we can be a little clever with encrypting the session file on disk, but it'll require some experimentation.