Fix stale navigation state issue (#520)

* Fix stale obs activity state

* Fix stale obs activity state

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Co-authored-by: Johannes Klein <johannes.t.klein@gmail.com>

Author: Chrischuck

src/navigation/BottomTabNavigator/index.js

@@ -168,7 +168,8 @@ const BottomTabs = () => {
         name="ObsDetails"
         component={ObsDetails}
         options={{
-          headerTitle: t( "Observation" )
+          headerTitle: t( "Observation" ),
+          unmountOnBlur: true
         }}
       />
       <Tab.Screen