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

package.json

@@ -68,6 +68,7 @@
     "expo-image-picker": "~14.1.1",
     "expo-localization": "~14.1.1",
     "expo-media-library": "~15.2.3",
+    "expo-sharing": "~11.2.2",
     "expo-splash-screen": "~0.18.1",
     "expo-status-bar": "~1.4.4",
     "expo-system-ui": "~2.2.1",