Persist user information across app launches

[romtsn]

Right now Sentry.setUser(user) is not persisted over app starts. Thus any captured events which occur at the app start (but before .setUser() is called) will result in an anonymous user.

This is already the case for AnrV2EventProcessor, we should do the same for MainEventProcessor but guard it with a flag. If this gets implemented around v7 milestone time, then we should include it there and enable the flag by default.

We should make sure this is not happening for plain Java Backend apps.