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/com/modals/Modal.web.tsx

@@ -97,6 +97,7 @@ function Modal({modal}: {modal: ModalIface}) {
               styles.container,
               isMobileWeb && styles.containerMobile,
               pal.view,
+              pal.border,
             ]}>
             {element}
           </View>
@@ -124,6 +125,7 @@ const styles = StyleSheet.create({
     paddingVertical: 20,
     paddingHorizontal: 24,
     borderRadius: 8,
+    borderWidth: 1,
   },
   containerMobile: {
     borderRadius: 0,