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/themes.ts

@@ -291,8 +291,8 @@ export const darkTheme: Theme = {
   palette: {
     ...defaultTheme.palette,
     default: {
-      background: colors.gray8,
-      backgroundLight: colors.gray6,
+      background: colors.black,
+      backgroundLight: colors.gray7,
       text: colors.white,
       textLight: colors.gray3,
       textInverted: colors.black,
@@ -307,7 +307,7 @@ export const darkTheme: Theme = {
       replyLineDot: colors.gray6,
       unreadNotifBg: colors.blue7,
       unreadNotifBorder: colors.blue6,
-      postCtrl: '#61657A',
+      postCtrl: '#707489',
       brandText: '#0085ff',
       emptyStateIcon: colors.gray4,
     },