UI lifecycle callbacks may be called when not signed in
If the user relaunches Briar from the recent apps list, activities may be recreated, started and resumed before
BriarActivity#onResume() checks whether the user's signed in and launches PasswordActivity.
Usually this only applies to NavDrawerActivity, because it's on the top of the stack when the user signs out. But if Briar's killed without signing out, other activities may be on the top of the stack when the user relaunches Briar.
Some of our lifecycle callbacks start operations that could throw exceptions if the user isn't signed in, as in #1482 (closed).