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/view/shell/Composer.web.tsx

@@ -32,7 +32,7 @@ export const Composer = observer(
 
     return (
       <View style={styles.mask}>
-        <View style={[styles.container, pal.view]}>
+        <View style={[styles.container, pal.view, pal.border]}>
           <ComposePost
             replyTo={replyTo}
             quote={quote}
@@ -63,5 +63,6 @@ const styles = StyleSheet.create({
     paddingHorizontal: 2,
     borderRadius: isMobileWeb ? 0 : 8,
     marginBottom: '10vh',
+    borderWidth: 1,
   },
 })