Improvements to the alt text behaviors in the composer (#910)

* Add an image preview in the alt modal

* Composer: add info about alt text and a green checkmark when done

* Shrink the alt visual indicator a bit so it doesnt obscure the image

* Fix typo

* Fix: avoid requiring multiple tabs to save alt text

* update react-native-screens

* Improve the alt text help tip

* Remove redundant hints

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Co-authored-by: Ansh Nanda <anshnanda10@gmail.com>

package.json

@@ -126,7 +126,7 @@
     "react-native-reanimated": "^3.3.0",
     "react-native-root-siblings": "^4.1.1",
     "react-native-safe-area-context": "^4.4.1",
-    "react-native-screens": "^3.13.1",
+    "react-native-screens": "^3.20.0",
     "react-native-splash-screen": "^3.3.0",
     "react-native-svg": "13.4.0",
     "react-native-url-polyfill": "^1.3.0",