Added: Add support for shared day/night theming across termux apps

With this commit, activities will automatically change theme between day/night if `night-mode` `termux.properties` is not set or is set to `system` without requiring app restart.

Dialog theming will be fully added in a later commit and may currently be in an inconsistent state or have crashes.

The `uiMode` has been removed from `configChanges` of `TermuxActivity`, this may cause termux app to restart if samsung DEX mode is changed, if it does, then users should report it so that it can be fixed by re-adding the value and ignoring the change inside `TermuxActivity.onConfigurationChanged()`. The docs don't state if its necessary. Check related pull request #1446.

Running `termux-reload-settings` will also restart `TermuxActivity`, the activity data should be preserved.