Android & visual fixes: color themes, repost icon, navigation, back handler, etc (#519)

* Switch android to use slide left/right animations on navigation

* Bump the repost icon down by a pixel

* Tune theme colors for contrast and darker bg on darkmode

* Move back handler to a point in the init flow that leads to more consistent capture of events

* Fix image share flow on android

* Fix lint

* Add todo about sharing not available

* Drop the android slide animation because it's too slow

* Fix 'flashes of white' in dark mode android

src/Navigation.tsx

@@ -6,6 +6,8 @@ import {
   createNavigationContainerRef,
   CommonActions,
   StackActions,
+  DefaultTheme,
+  DarkTheme,
 } from '@react-navigation/native'
 import {createNativeStackNavigator} from '@react-navigation/native-stack'
 import {createBottomTabNavigator} from '@react-navigation/bottom-tabs'
@@ -256,8 +258,9 @@ const LINKING = {
 }
 
 function RoutesContainer({children}: React.PropsWithChildren<{}>) {
+  const theme = useColorSchemeStyle(DefaultTheme, DarkTheme)
   return (
-    <NavigationContainer ref={navigationRef} linking={LINKING}>
+    <NavigationContainer ref={navigationRef} linking={LINKING} theme={theme}>
       {children}
     </NavigationContainer>
   )
@@ -334,7 +337,7 @@ const styles = StyleSheet.create({
     backgroundColor: colors.black,
   },
   bgLight: {
-    backgroundColor: colors.gray1,
+    backgroundColor: colors.white,
   },
 })