Summary:
On Android 10 whole activity is recreated when the user changes system theme. We should prevent it by adding `uiMode` in `android:configChanges` key in the default template.
## Changelog
[Android] [Fixed] - Android 10: Prevent activity recreation on theme change
Pull Request resolved: https://github.com/facebook/react-native/pull/26413
Test Plan: NOOP
Differential Revision: D17342811
Pulled By: cpojer
fbshipit-source-id: 2bed9b5f91e1b67451fefe34b02a1129d6c80d15