[native] Prevent other nav actions from messing with LOG_OUT
[native] Prevent other nav actions from messing with LOG_OUT

Summary:
While testing an unrelated issue, I noticed that if I initiated a nav action between `LOG_OUT_STARTED` and `LOG_OUT_SUCCESS` Redux actions, that nav action could occur after the `LOG_OUT` navigation action. This would cause the focus to come back to the `AppNavigator`, making it look like the user logged back in, but without actually showing any user information since there is no user logged in in Redux.

This diff:

1. Implements `getStateForRouteFocus` to prevent an action affecting `AppNavigator` from giving that navigator focus if we're logged out.
2. Make sure that the navigation state is reset before the `LOG_IN` navigation action, in case it got messed with after the `LOG_OUT` navigation action.

Reviewers: palys-swm

Reviewed By: palys-swm

Subscribers: KatPo, zrebcu411, Adrian

Differential Revision: https://phabricator.ashoat.com/D85

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