Simplify opening app check for exisiting observation

[ErikSin]

Currently when a user opens the app we check for the existence of an observation in persisted state. If an observation exists, we use getInitailRouteName to navigate the user of the Observation Create/Edit Screen.

Due to this, we hve to derive the navigation stack to have the home screen in it so the user can go back to the home screen.

To simplify this, we can just check for the existence of an observation in the home screen. If there is an observation, then navigate to the Observation Create/Edit Screen.

To Do:

• Add useEffect to home (Map) Screen
• add appropriate logic fromgetInitialRouteName into this useEffect
• remove initialRouteName from RootStackNavigator (the onboarding screen are conditionally rendered anyways)
• remove the derived navigation states