sharing: Switch to 'react-navigation-tabs' for creating tab bars.

Since the RN 0.60 upgrade, using `createMaterialTopTabNavigator`
exported by 'react-navigation' causes an error - that
'ViewPagerAndroid has been removed from React Native'. Switching to
the one exported by 'react-navigation-tabs' fixes this.

Author: agrawal-d

src/sharing/SharingScreen.js

@@ -71,7 +71,12 @@ class SharingScreen extends PureComponent<Props> {
     }
 
     return (
-      <Screen canGoBack={false} title="Share on Zulip" shouldShowLoadingBanner={false}>
+      <Screen
+        canGoBack={false}
+        title="Share on Zulip"
+        shouldShowLoadingBanner={false}
+        keyboardShouldPersistTaps="always"
+      >
         <SharingTopTabNavigator navigation={navigation} />
       </Screen>
     );