Include sign-in time in crash reports
Currently, we include the app start time in crash reports, but we register a boot receiver in order to show the sign-in reminder, so the application instance may be created shortly after boot, even if the user doesn't launch the app until much later. It would also be worth recording a launch time (e.g. the first time an activity's created) or a sign-in time (e.g. the time when BriarService starts).