android: Make dark-theme support official, in styles.xml.

As Android's dark theme guide says is necessary [1].

Done now, to follow the template-app change in
facebook/react-native@2b56011f9, on the path to the RN v0.64
upgrade.

The RN commit contains other changes to the template app that we
don't transfer to our project right now:

- The placeholder App.js starts using `useColorScheme` [2], an API
  that was new in RN v0.62. We may want to use this API in some
  places, during or after zulip#4009, but it doesn't seem necessary now.

- On iOS, the root view defined in AppDelegate.m gets its
  `.backgroundColor` set to `[UIColor systemBackgroundColor]` when
  possible. That seems intended to vary the background with the
  system-set theme (dark or light). We don't want to make this
  change: we've so far decided to set `.loadingView` instead of
  `.backgroundColor`, and we do so in a way that explicitly doesn't
  change with the theme (it uses the brand color).

- On iOS, the launch-screen storyboard is changed to adapt to the
  system-set theme. No need to make that change; we've so far chosen
  not to have it change with the theme, and just to use our brand
  color.

[1] https://developer.android.com/guide/topics/ui/look-and-feel/darktheme#support-dark-theme
[2] https://reactnative.dev/docs/usecolorscheme

android/app/src/main/res/values/styles.xml

@@ -1,7 +1,7 @@
 <resources>
 
     <!-- Base application theme. -->
-    <style name="AppTheme" parent="Theme.AppCompat.Light.NoActionBar">
+    <style name="AppTheme" parent="Theme.AppCompat.DayNight.NoActionBar">
         <!-- Customize your theme here. -->
         <item name="colorPrimary">@color/primaryColor</item>
         <item name="colorPrimaryDark">@color/primaryColorDark</item>