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/lib/routes/back-handler.ts

@@ -1,11 +1,8 @@
 import {BackHandler} from 'react-native'
 import {RootStoreModel} from 'state/index'
 
-export function onBack(cb: () => boolean): () => void {
-  const subscription = BackHandler.addEventListener('hardwareBackPress', cb)
-  return () => subscription.remove()
-}
-
 export function init(store: RootStoreModel) {
-  onBack(() => store.shell.closeAnyActiveElement())
+  BackHandler.addEventListener('hardwareBackPress', () => {
+    return store.shell.closeAnyActiveElement()
+  })
 }